I'd suggest that enslaved clerics or other casters would be unable to summon good summons 'til they are freed.
And that paladins and other classes that depends on a good god would lose their connection to their powers 'til they are freed.
Doesn't make sense the gods would allow their power being used for evil even if their chosen champion is doing it against their will.
Would make being enslaved meaningsful and make people want to escape.
Plenty of examples of this in d&d books and novels.
