I do believe that there is some complications for allowing an add on to be modable because it is already a type of mod. However im not sure if this will prevent CZ mods from happening. though I intend to find out as soon as I can to answer your question.
We are currently using a Ritual only version of Radiant that I happen to prefer over GTKR currently due to some clunky interface issues and lack of paint select. hehe though I do know ill have to bite the bullet and use GTKR if I intend to make anymore Quake 3 dm maps this year. Im sure I can at least get paint select working with the latest version of GTKR and that would make my world into blue happy skies. Im sure that shortly after CZ is released some mad man like Ydnar will add support for CZ to GTKR.
This should be good news to all those people out there who love Radiant and want to make levels for CS or CZ or even just simple Half life.