Nice work! Short answer is you can't yet (I know, not what you wanted to hear). We're not crazy about the internal API yet, and we haven't settled on a registration mechanism. I don't have an ETA, and I can't promise it will make GWT 2.1 (although you've certainly just made the need a lot more apparent).
A nit: we've tried to keep a convention of using lower case names for XML elements that don't correspond to runtime objects, so <l:BoxLayoutData> should probably be <l:boxLayoutData>. If you want to get this out the door in the meantime, I'm afraid you'll have to copy and paste UiBinderGenerator and UiBinderWriter, and provide some gwt.xml magic to let your generator run instead of ours for GWT.create calls on UiBinder.class. Another alternative is to do some classpath cheating. Patch UiBinderWriter to register your parsers, and put your version in the Mosaic jar. If your users put your jar in the classpath before gwt's, you should be good to go. rjrjr
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