I suppose this isn't a good time to comment that I have broadly
similar code -- vrrp-style packet format, IP only, no layer-2
frobbing-- that's been working for about a year now?

I really, really would prefer not to see this in the tree (which is
why I never submitted my version).

The Right Way to do this is to go through the IETF: write up a spec,
get a new IP-multicast or a new protocol assigned.  (While you're at
it, increase the id space to 16 bits, if not 32, and bump the version
number.)

If you don't want to deal with the ITEF, obtaining a new local-subnet
multicast address from the IANA should be straightforward, and needn't
involve the IETF at all.