Cool!
Are you following
BIP10?
I didn't know about this BIP. I looked at it and at first glance it seems good: there is a transaction that needs to be signed. But I don't think it will work for this use case: every user wants to add transaction inputs to this, not just sign them. And they need to be signed using "anyonecanpay".
I would talk to etotheipi about this, and try to use essentially the same format. The details are slightly different, but the idea is very similar: you are collecting signatures for later combination.