DDSSEC13 — Provide a way for multiple vendors to extend the algorithms and interoperate

The specification provides mechanisms to extend the set of cryptographic algorithms in future revisions.
The specification also provides mechanisms to extend the set of cryptographic algorithms by a single vendor without requiring a revision of the standard. Those extended algorithms will be understood by the vendor and ignored by others.
What is not clear is if two vendors can agree to use another cryptographic algorithm and interoperate. This use-case may be relevant to an end-user that is using multiple vendors and wants to have those vendors work together to use a new crypto algorithm

This issue requests some mechanism to support this last use case.