Most Gods worshiped in Ravenloft might very well be the Dark Powers in disguise. As I recall, in order to reach their faithful in the Demiplane, a God needs to send a servitor (a Solar or equivalent, able to give out spells) there, at which point they are trapped forever (and probably constantly tormented/tempted to fall by said Dark Powers).
If the first novel is still canon, then Lathander of the Realms presumably has reach in at least Barovia. Being a NG (when he isn't in his cranky LN persona), Lathander would probably be happy to grant powers to any Good (and possibly True Neutral) Divine caster.
That's about all I can really guess at, however. Outer Planar beings can have their connection to their home planes severed, losing some of their powers, but being warped in other ways (Inajira seemed to function, but then there's Baltoi, the 20' long (and 24 Hit Die!) Marilith encountered in RE1, Adam's Wrath, who lost her magic, but grew a size category as a result of her exposure to the Demiplane).