The short answer is that Openwave's WML Extensions can be used if you target your code for the UP.Browser version 4 or higher.

The slightly longer answer is that according to the WAP specifications, every gateway should pass unrecognized mark-up along to the browser unencoded. This has been seen to work on common gateways used by network operators in GSM areas (UP.Link, Ericsson, Nokia). If you target your application for the UP.Browser (i.e. if you use multiple version of your application), you will be able to exploit the possibilities offered by the extensions.