This is not a full answer, but I do have the JAXP 1.22 version of Xerces from Sun's WSDK 1.1 installed and working. Don't forget to add the following system properties to your JBoss startup script:
I have not experienced problems relating to hardcoded Crimson references (not that that fully proves anything).
Also, for Xerces 2 there are 2 jar files (xercesImpl.jar and xmlParserAPIs.jar), not one, and two files to replace (crimson.jar and jaxp.jar).
If you use the WSDK, move out the same two files and move in the following 5 jars:
You apparently don't need to copy in jaxp-api.jar
I also moved xalan.jar out of $JBOSS_HOME/server/[instance]/lib.
Hope this is useful. If you've already covered all this, then my apologies for wasting your time ;-)
The chances of backporting the changes detailed in the note to 3.0.x are slim. However, I think you will only be affected by this in 3.0.x series if you're using the JBossMX ModelMBean implementation.