I’m wondering if any escrow services on Bitcointalk support milestone-based payment releases instead of a single full release at the end.
I don't think there's going to be any issues with the partial release of funds upon completion of each of the stages.
If yes, which trusted escrow providers currently offer this, and how do they usually structure dispute handling between milestones?
It'd be better if you ask these questions from the escrow providers themselves
[especially for the latter part]:
Few Trusted Escrow Providers- FWIW, I prefer not to handle it due to the extent of issues that might arise in the development stage (I had a very bad experience in the past), but I'm sure you'll find someone from the above list.