Yes. The best "playable" undead race is the necropolitan, found in (I believe) Libris Mortis. This template is the best choice for a PC because it has no level adjustment, merely causing the character in question to lose a level. Therefore, any build is possible with this basis, with especial attention being paid to the taint rules from Heroes of Horror.
Beyond that, look in Savage species for a way to make templates for any creature of your choice and many interesting exotic (and some undead) races. Libris Mortis also has some interesting undead rules.
The aasimar alternate racial trait scion of humanity can't grant access to human alternate racial traits
Up front: Don't confuse race traits with racial traits. Human (and aasimar) race traits are what the aasimar now qualifies for because of the alternate racial trait scion of humanity. On the other hand, racial traits are those specific facets of the creature's race that can be picked and traded for other facets of the creature's race because of the creature's race.
So, although the aasiamar has picked the alternate racial trait scion of humanity, the aasimar's race nonetheless remains aasiamar. Hence, an aasiamar that takes the alternate racial trait scion of humanity can no more pick human racial traits than a half-orc can pick human racial traits. An aasimar with the alternate racial trait scion of humanity could take as its two race traits Aspiring Bard and Bralani's Step, but the same aasimar couldn't take the human alternate racial traits heart of the sea and heroic.
So while the alternate racial trait scion of humanity says
Some aasimars' heavenly ancestry is extremely distant. An aasimar with this racial trait counts as an outsider (native) and a humanoid (human) for any effect related to race, including feat prerequisites and spells that affect humanoids. She can pass for human without using the Disguise skill. This racial trait replaces the Celestial language and alters the native subtype.
...this alternate racial trait doesn't give the aasimar anything the aasimar didn't already possess. Herolab shouldn't've allowed such an aasimar to pick the human alternate racial trait dual talent, first, because the aasimar's not a human, and, second, because the aasimar doesn't have the things the alternate racial trait dual talent needs to replace. That is, the alternate racial trait dual talent says that it
replaces the +2 bonus to any one ability score, the bonus feat, and the skilled traits.
And the aasimar, even if the GM allows the aasimar to take human alternate racial traits, doesn't have those other traits to trade away for this trait, making the aasimar ineligible for the new human trait, just as the aasimar would be ineligible for a feat that had as a prerequisite human and Str 70 (assuming the aasimar didn't have Str 70, obviously). Even were the aasimar to meet the human prerequisite—which it does—, it can't meet the remaining prerequisites.
Aasimar variants' ability score bonuses replace the typical aasimar ability score bonuses
That is, an aasimar gets either the typical ability score bonuses or the ability score bonuses for the variant he rolls or picks:
Players may choose one of the following six heritages for their aasimar characters in place of the traditional aasimar racial features. Each heritage presents new ability modifiers, spell-like abilities, and skill modifiers that replace the default aasimar racial traits, as well as a pair of custom traits. Each entry also discusses the most common (though by no means ubiquitous) personality traits, physical features, and places of origin of aasimars with that particular heritage.
Emphasis mine.
Best Answer
Yes, Rakshasa player characters are supported! ...sort of.
The Monster Manual (page 212) has the typical monster manual "RAKSHASA AS CHARACTERS" racial stats block: they start play as level 14 characters (7HD and a +7 LA) with an effective seven levels of sorcerer built into the character (future class levels of sorcerer stack with the racial levels). However, it's important to remember that many of the "_ AS CHARACTERS" stat blocks are more suited for the DM's use in making NPCs; while technically they're viable as player characters, it may not be a satisfactory experience. The DMG1 (page 172) notes that "most monsters are not suitable as PCs" even when joining other characters who are at their effective level after adjustments. Whether the Rakshasa is overpowered (providing seven levels of caster and some sweet monster features) or underpowered (half-level casting of a non-optimal casting class and some monster features which may not compensate) I leave as an exercise for the individual group, but it is wonky.
With that in mind we can now look at the 3.0 Savage Species book (page 191). It contains a "racial class" gimmick which lets you start as a level 1 Rakshasa with minimal features and slowly level up as a Rakshasa. At level 14 you become a full-fledged Rakshasa and from 15 on you start taking regular classes.
Although Savage Species is 3.0, it was never re-released for 3.5--only errata'd, and very minorly at that--and technically that means Wizards considers the book's contents (with errata) are considered 3.5-compliant. In practice, the SS Rakshasa is somewhat less balanced and I'd suggest anyone using it to work with their GM to tweak its features to realign them with the 3.5 version of the creature. For example, 3.5 Rakshasa have high spell resistance while 3.0 Rakshasa get immunity to spells of incrementally higher levels. Spell immunity is a messy business and overall I find the spell resistance version is easier and more balanced.