WML is just carried on HTTP so it is not a question of the capabilities of JBoss holding back development.
Some people take a "pipes and filters" architectural approach to rendering WML - to give flexibility for generating WML or HTML from the same XML stream.
I prefer getting closer to the data source - direct coupling with the EJBs and having flexibility to generate for the capability of the WAP browser. Also, since the WML page content is much smaller, you get some efficiencies over rendering an entire format from XML that can suit HTML, down to the WML content. I use the OpenWave tag libraries and rendering libraries. They can use advanced features of WAP browsers as on Siemens mobiles, or degrade the output to support less capable browsers such as on Nokia mobiles. You get a really nice user experience using the OpenWave advanced features.