If I have understood correct what others have had success with you should be able to get this behaviour by setting the MDB pool size to 1. Look into the manual how to do advanced config of MDB.
Indeed, you were correct. The docs weren't very clear on this, and I still don't understand what all the elements of standardjboss.xml actually do (but sure would like to ;), but I was able to limit the number of MDB instances to one by changing the value to 1 under <container-invoker-conf>.
It did not solve my overall problem of slow INSERT times, but it did accomplish the smaller goal. Thanks.