That might be interesting, but then any protocol extensions would
require IETF updates. I'm not sure that's ideal. Currently nbd is a
Linux-internal protocol, and we can change it at will; we'd lose that
ability to some extent. Whether that's a fatal problem is, of course, up
for discussion.
Having said that, I do agree that having a more formal description of
the protocol is a good thing; proto.txt is a good start, but it's not
fully there yet.
At any rate, if there's going to be an IETF draft, I'm going to be a
(co-)author ;-)