Hi Geoff,
Perhaps I may be permitted to add some background data here.
Ofcourse! Thank you :)
My interpretation of the relative roles of the RIRs, IANA and the IETF in this area is that such reservations, as distinct from allocations or assignments, should be performed by the IANA, and for IANA to undertake this it should be part of the protocol's address plan, and hence should be published as an RFC. i.e. as far as I understand the situation such reservations of number resources for documentation purposes falls to IANA to perform, and the instructions to IANA are in the form of a published RFC. The logical inference is that such proposals should head to the IETF rather than the RIRs.
I had the same feeling, but it's good to get this background information. Thanks, Sander