sergiol has great passion for this possibility, as of 2006.
Quite a few people (PT, KBK, DKF, tclguy, AK, ...) have made cogent observations about Tcl's channel and message semantics, how to properly generalize them to make a place for UDP, and so on.
While CL would appreciate the aesthetic advance represented by UDP in the core, and certainly would make enthusiastic use of it, he's already a satisfied customer of the existing alternatives for UDP for Tcl, and, despite rumors to the contrary, unlikely to be in a position to help significantly.