No. As per the Demon Storytellers Companion (p.69):
Because of their undead nature, vampires cannot benefit from pacts of Faith, nor can they be possessed by a demon. A mortal who is bound to a vampire can be enthralled by a demon (and vice versa!), however, and she may be possessed like any other human if her Willpower is low enough.
The latter part says that a Ghoul can be possessed, but it's a bit vague on what happens if it's possible to possess a Revenant. If it is, this likely does not result in them retaining their Disciplines much like how a Demon does not have access to the Spheres of Mages and Edges of Hunters that a Demon possesses. And while Demons can make pacts with Garou, they cannot possess them so there's no potential gain in power here.
Clearly any techniques of spy craft will work on any vampires in the same way they work on humans: ideology, shame, revenge, and greed pretty much cover it all. These techniques are out of scope but you can be found easily. there are literally hundreds of such methods used everyday in real life.
One of the most common might be the "enemy of my enemy is my friend": So, group A and B are both Sabbat and both hate each other. Prince Y of the Camarilla contacts A and seeks a temporary alliance to mess B up. What could go wrong⸮…
Vampire have access to mystical powers: Dominate and Presence, used both in conjuncture, can achieve complete control over a person in such a way that the person does not remember being controlled. Thus, a vampire could condition another to write a journal of what they are doing. A repository of what they feel, what they do, who their friends are, and hopes for the future. The writer then sends a copy to their master and forget they did so. A perfect spy who does not even know they are a spy.
Note that RAW does not quiet support this: You can make it work but it is akin to putting a round peg in an oval hole. In my view, disciplines were always more than discreet powers and their use could be morphed with enough mastery and training. Your mileage might vary.
With this in mind, a sleeper personality might be implanted in a young vampire, ready to take over on the right trigger a la Manchurian Candidate.
To answer the first part, I am not aware of any sleeper in either sects within the published material. However, it's been years since I read them so could not be remembering.
And because a good conspiracy theory is always welcome: You know that the Sabbat and the Camarilla were created by the Elders as tools to distract the younger vampires from finding out they are just food? Thus it is entirely possible that the Vaulderie is not as efficient at breaking blood bounds than is believed… Some elders within each organisation have a good knowledge of what is happening in the other.
Best Answer
The best answer is this:
This is from unofficial white wolf wiki, but official book "Guide to Camarilla" from revised edition says exactly the same on page 53:
When you are speaking of "Tzimisce antitribu", you mean "Camarilla". No, it can only be a super rare temporary alliance. And the Old Clan has no connections to Camarilla. Check Hedeon Yaroslavich, for example. Of cause, the golden rule of storytelling applies here... But just don't do it. There are many interesting ways and stories, much better then "let's imagine Tzimisce antitribu".
And - nope, I haven't seen any such canonical characters.