There is not much that can go wrong when using a port number without registering it, especially when the receiving application accepts data only from a pre-configured peer. The port numbers used essentially don't matter in that case, the only thing you need to make sure is that you don't have another session with that same peer that would want to use the same port number.
In my cut-and-replace version of RFC 1226 for UDP, I mentioned 93 as a default since that's the JNOS default. But perhaps we don't need to mention a port number at all. Or perhaps we could just say something like:
"The UDP port numbers 93 and 10093 are commonly used for AXUDP. But any port agreed to by the administrators of the two end-points can be used."
I tend to lean toward mentioning what is common since it helps out the newbie to know what will be expected from others that he/she connects to.
Michael N6MEF