Good to see so much activity! But please do remember, there's more to "multisig" than just keys - you need the whole user experience to be planned out and specced for fully interoperable implementations.

For the "group account for an organisation" feature, you don't really want to expose end users to the notion of a key. Historically this has just led to confusion (and an ugly visual explosion of padlocks and small metal objects :) It'd be much better for the UI to be designed in terms of people, perhaps with a bit of social network integration to avoid having to set up profiles, and then a "group spend" feature would behind the scenes rendezvous with the others and swap signatures around, etc. So for interop, you'd need to define all the rendezvous protocols as well.

CoinVault is also using a partially signed transaction format whereby 0-length placeholders are used for missing signatures in the transaction scripts.

I don't know how you are implementing this/what framework you're using, but I suggest using placeholders that are the length of an actual expected signature, at least when forming the transaction. This is what bitcoinj will do because otherwise you could end up miscalculating the fee, which is based on the final size. See TransactionSignature.dummy() in the API.