Oh yes the other thing we need to decide is how to extend BIP70.

Protocol buffers have an extend keyword. But I'm not sure it's what we really want. IMHO a simpler solution is to have a single "living" version of the protobuf (where? in a new git repo?) which has all the fields defined by all the accepted BIPs in a single place. Otherwise the build process and so on for wallet implementors would get unnecessarily complicated for no real reason. Also if you wanted to pick a new number for fields, you'd end up having to read all BIPs to figure out what is available. Simpler seems better.

If we do that then I suggest just using field number 8 or 9 or whatever rather than 1000. IMHO fields 1000+ should be for private extensions that are unlikely to collide with other users.