As I understand it, by RAI, there's no way out of Ravenloft other than the Dark Powers letting you out, such as via a portal or the mists just transporting you out. I'm wondering if it's possible to get out via RAW without the Dark Powers' permission. Can you Iron Heart Surge the mists, for example? I'm looking for a solution that's within RAW in either 3.5 or Pathfinder, even if it's cheesier than Pun-Pun.
[RPG] How to escape Ravenloft in spite of the dark powers
They are some form of super-deities, or perhaps a personification of some multiversal axiom about true evil. They are never described in character/deity terms, and in fact have imprisoned some impressively powerful beings, including a couple deities from other D&D game worlds. (Vecna, specifically, being the most noted, but there was another.)
The Dark Powers are noted only by what they do, and that is, they reward evil acts with more power, but at a price of less freedom. Sufficiently corrupt individuals find a chunk of the demiplane altered to become their own personal hell... LOADS of power, but trapped forever there, and unable to get whatever it is they truly desired in life.
The Dark Powers also search the multiverse (or at least the Prime Material Plane) for certain "worthy" individuals, and set the Mists upon them. If they don't flee the Mists, anyone near the victim also is transported. The area is removed from where it was, and attached somewhere in the demiplane.
It is worth noting that they control the demiplane, and can affect all within it. They can not, however, prevent Vecna from granting spells nor communing with his clerics, but they can prevent him from making personal appearances. They have moved regions about, deleted regions, and made and deleted borders on an apparent whim.
The Dark Powers do not use manifestation forms, do not reveal themselves if they use mortal forms, do not grant spells, are not subject to commune spells. Even calling them entities or an entity is a stretch.
Source Note: It is worth noting that the nature of the Mists grabbing parts of the Prime Plane was the focus of one of the TSR Retail Play modules for AD&D 2E; in that same module, PC's who do not flee the Mists wind up in Ravenloft as well. The demi-plane does not provide inhabitants; it captures them with the evil being.
In the 3E materials from S&S, it's noted that at least one domain dark-lord was originally from the Demi-plane. Another was in the demi-plane already when the Dark Powers noticed them.
It Depends on the Darklord
Not all Darklords are also the temporal rulers of their Domains (though some are), and not all of them reveal or flaunt their true natures. Indeed, some Darklords are deliberately denied political rulership of their domains (such as the Darklord Adam) as a way to isolate them and make them lonely. Of those who conceal their true natures, some do it out of habit or convenience (Azalin, Strahd), others out of necessity (Harkan Lucas, the Hive Queen) and still others because they hate what they are or have become (the ruler of Saragoss is one example). Oftentimes a Darklord still has feeding requirements, peccadilloes, or both that are easier to fulfill if the populace doesn't know what they are.
In Barovia's Case
According to Darklords and the Ravenloft Campaign Setting none of the citizens of Barovia believe Strahd to be human, but most of them have no idea what he truly is. They refer to him as "The devil Strahd" and fear him greatly; he rules with a casually brutal hand, ignoring them except when they get out of line. The Vistani know what he is, but Strahd long ago cut a deal with those wanderers and they are unlikely to inform others of his nature - let alone help them - without being paid the right price.
As noted in the Ravenloft Campaign Setting, education standards in Ravenloft are pretty low. With a few rare exceptions, it is a rural place without major centers of learning (a notable exception is the massive domain of Darkon, ruled by Azalin, with several urban centers) and riddled with racism and xenophobia. Often natives of Ravenloft will identify anyone unusual - a non-human race, magical abilities - as being "evil" and react with terror or, worse, a lynch mob. Even if you told one point-blank that the local Darklord was a vampire they might have no true idea what that signifies and simply attempt to protect themselves with ineffective superstition. Help from the locals is not a likely thing and might end up harming everyone involved; would-be heroes are not advised to seek it.
RAI compliant: Major Artifacts can beat Ravenloft's travel restrictions. (Ravenloft PHB p.123):
Planar Bubble (Sor/Wiz7, Clr7, SpC; or Planar Shepherd class feature):
This makes your surroundings obey the planar traits of your native plane instead of wherever you actually are. So obtain a creature that's native to somewhere other than Ravenloft (via a calling spell if you yourself aren't), cast Planar Bubble on it, and then leave via ordinary Plane Shift. As an abjuration spell that isn't about banishment, Ravenloft doesn't by default corrupt the casting of Planar Bubble; and once the Planar Bubble is active, Ravenloft can't mess with your subsequent spells.
Precipitate Complete Breach (Sor/Wiz9, Planar Handbook; or Anarchic Initiate class feature):
As this is a teleportation spell, it is subject to Ravenloft's rule that "no spell can transport anyone or anything across a closed domain border or out of Ravenloft" (Ravenloft PHB p.109). So this only works under the interpretation that the spell isn't transporting you; the spell is making two planes coexistent even if they weren't connected before, and then the transport is done by ordinary walking.
Gatecrasher 3 (PrC, Manual of the Planes) or Silver Key 10 (PrC, Dragonmarked):
If there are portals out of Ravenloft, and if you can find one, then these classes let you use the portal without the Dark Powers' permission. Gatecrasher in particular can explicitly beat portals that were sealed even by deities.
Find The Path (Clr6) can guide you to such a portal, "provided you know what you're looking for" (Ravenloft PHB p.112). Obtaining a precise enough rumor to start searching, however, may still be difficult.
While you should never cast Wish in Ravenloft (it's always either twisted or fails entirely even if you wish for something that's safe under the core rules), someone who's already outside could use the "transport travelers [...] regardless of local conditions" mode to rescue other people.