I think there is a larger problem/better solution that could be addressed here. I'm not sure how everything is connected together in your mind but I see only the following problems with using external js libraries:
-) Licensing - if it fits...
-) Reliance on 3rd parties can sometimes be dangerous.
-) Assets aren't overridable? I swear I keep remembering fleeting glances of global-asset or something like that. If they aren't somehow able to be packaged and then overriden by people it's probably a good feature to address anyways.
The even connection mechanism is really elegantly designed. Want to connect running function X with a user moving their mouse over element A? dojo.event.connect(elementA, "onmouseover", "functionName"); I can't explain all of the different ways things can be connected (even aspect sort of stuff) but it's here: http://dojotoolkit.org/docs/dojo_event_system.html.
Validation/Date Pickers/All of the form connection stuff/Pallette/etc. could all be replaced by very robust and easy to debug/unit test/visualize code. It's probably a 4.1 type of change, but I'm chomping at the bit to bring some of this stuff into tapestry so you wouldn't have to do it alone