Would really like for the devs to weigh in on this
They don't respond to forum posts very often.
If the way DOS/DOS 2 were created is any indication, custom characters in the story campaign are unlikely, because characters cannot be separated from the campaign. If Shadowheart A solves quest A by killing the village and she is replaced by Shadowheart B from a different campaign who didn't slaughter the village, how can the actions be resolved? How does that affect the character development?
Moving custom characters around in GM campaigns might be another story. It's hard to say.