Well yes, that is definitely understandable why importing isn’t an option.
I’m not sure the best suggestion at this point as I don’t know how your characters are setup. I tried a duplicate special of one of our characters and it all worked as far as I can tell. But our drawing overrides are connected to controls, if we use them…so maybe there is less chance for them to go haywire as they are being explicitly set.
For what its worth, I am sure you know that the attribute spreadsheet is a great way to change to drawing overrides on as many objects as you feel like selecting. Don’t forget that there are drawing overrides on the transforms as well as shapes of most nodes…just to complicate matters.
The last suggestion to help troubleshoot this kind of maya-wonkery is to create a duplicate special cleanup script. As long as you have a solid naming convention it shouldn’t take too much time to have a script to go through and make sure everything is set the same as the source rig.
We have cleanup scripts like this for every step in the pipeline because there are so many things that can pop up.
Sorry I don’t have a more definite “helpful” suggestion.
Maintaining render layer membership stuff is annoying.
Good luck!
~Ben