Ways of Getting Around Canon...

[IanFordam]

As for not using the "born in Darkon, taken away as a baby" loophole that can't be used often:
As long as you use it for a different campaign each time, I don't see the problem.

Once per campaign strikes me as not too often.

[Galeros]

I edit a lot of things about Ravenloft. Fiends, for example are formed from the Demiplane itself and are not Outsiders. This was presented as an option in VRGTF, but I do not see it used often, or ever really. Fiends from the rest of the multiverse can still get stuck in the Demiplane, but it is not common, and native Fiends hate them even more than they do the locals.

[The Lesser Evil]

An interesting variant on the Darkonian claiming thing might be to require the foreigner to achieve some kind of blending into Darkonian society, which could be one or a combination of being accepted into the community and/or coming to see one's self as part of the community.

[alhoon]

I edit a lot of things about Ravenloft. Fiends, for example are formed from the Demiplane itself and are not Outsiders. This was presented as an option in VRGTF, but I do not see it used often, or ever really. Fiends from the rest of the multiverse can still get stuck in the Demiplane, but it is not common, and native Fiends hate them even more than they do the locals.

I think there are a few fiends that are mist-subtype, really.

[Wolfglide of the Fraternity]

There are also some fiends that should of had the subtype, but it wasn't listed properly. I believe the errata documents suggest that the assassin imp is a Mists-subtype fiend, and not a true outsider.