Seems like a reasonable RR-type to add, given that it serves a functional purpose in the world and using RFC3597 for this, while possible, would be a PITA. I haven't had much time to work on gdnsd the past couple of months, sadly, but I'll put it on the list of relatively-trivial things to try to squeeze into the next feature release.
Alternatively, if someone else wants to work it out and file a pull request against master, that would be awesome too! The only potentially-complicated bits I foresee here aside from copying and modifying the related code for some existing RR type are: (1) the numeric id for this is >256 and (2) not breaking anyone who's currently using RFC3597 to specify this RR-type (the code assumes no overlap between explicitly-supported RR-types and RFC3597, but clearly that was a bad idea for future-proofing).
-- Brandon