I traced through the jboss code and found the problem. It appears that the managed connection factory stores drivers in a map of some sort using the url as the key. In my case I had 2 ds.xml files deployed with different jndi names, but the same url. One of them had the actual OracleDriver specified and was stomping on my proxied driver config. The example from the wiki works great now.
fyi, unless there's some really compelling reason that I'm not aware of, I'd change the managed connection factories to store things with a more unique key. I can imagine situations where using more than one datasource config trying to talk to the same url could reasonably happen. If possible getting the jndi name and just appending it to the url string for the purposes of using it as a key would probably work.