Beside this thread, I have also been communication, via mail, with a guy from IBM. He says that the resource adaptor is the only supported approach for using WMQ within the JBoss environment.
I suppose there is some inconsistency in how IBM handles this case. I have personally witnessed (both via email and phone) IBM supporting JBoss AS integration with WSMQ without their JCA RA (i.e. connecting directly to WSMQ using a JMS bridge configured to use a JMSProviderLoader).