Francois,
The UUIE _is_ a Q.931 IE, 5.8.6.2/Q.931 does not exclude this IE, and neither H.225.0 or H.323 address the issue of how to handle a UUIE--or any other IE, for that matter--with invalid content, therefore the behavior defined in Q.931 is normative for H.323.
An "invalid coding" for an information element does NOT include the content of a User-User information element. By definition, a Q.931 User-user IE can include whatever you feel like. A proper implementation will not reject a call because of the content of the User-user IE.
_All_ of Q.931 is an "ISDN thing," so what's your point?
Finally, there is much in Q.931 that H.323 does not reference, but that does not invalidate the parts of Q.931 that H.323 references.
H.323 does not use Q.931. H.225.0 recycled some of the messages (but not all of them). More importantly, it doesn't explicitly use the PROCEDURES of Q.931. There are some stringent timer requirements for example in Q.931 that not used use by H.323. The DISCONNECT and RELEASE messages are not even used. We are not even setting up a bearer channel in H.323 when doing a call setup.
In summary, H.225.0 is NOT Q.931.