The Right Way™️ to do this is definitely to define two new ACME Identifier types, have the client present them in the NewOrder request, and have the server reflect them in the Order object and its associated Authorization objects.
That said, you don't have to write an RFC to do this. The IETF adopts RFCs that represent "rough consensus and running code", so even if you did write an RFC, you'd be implementing this solution before that document is finalized. So just go ahead and implement it! Then if you think it is useful enough to be standardized, put forward a draft RFC (and yes, this one is a good example of what it would look like) so that others can benefit from your work.