No. You cannot prepare a non-xaconnection so it cannot take part in two phase commit.
There is an implementation of the "Last Resource Gambit" in latest jboss4
but it is still being tested. Perhaps you would like to help test it?
I'd like to help to test it out in JBoss 3.2.3. I do not have Jboss 4.0 documentation and have not tried it out yet.
Let me know if I can be help.