I think in order to be able to have a replacement category there would have to be some serious rules as to who is eligible, to make it possible for nominators to see them. I would perhaps limit it to leading roles for a start (they could always expand that to featured if it works a few years later), and I would distinguish between male and female. They would have to be in the show for a certain amount of time, say at least 3 months or even more. And perhaps there could be something like: they can only invite the nominators and voters back to the show once per season to see who they want them to see. That way, they can submit several people from their show (say, Wicked can submit 1 Elphaba and Glinda, not just one actor per show), but it would keep the amount of shows the nominators have to see at a minimum. It's really not that many if you think about it that way. Exclude all news shows, most plays because they close before they have any replacements, and some shows from last season that still have the OBC (e.g. Something Rotten). You're left with 10-15 shows tops I think that they would have to see over the course of the season. They could even reinstate an additional rule of not being allowed to invited during March/April when they are busy with new shows anyway.
It's not ideal and just some thoughts from the top of my head, but I do think it could work and would be a good start to recognize replacements, and might encourage name actors to replace more often, which never really happens...
I agree about an ensemble award, this one is very necessary!
I like the Music Director one, but I feel like that's another like sound design where nobody knows how to judge that...