If I'm understanding this right, you want two methods of input.
First, the 50 attributes manually filled in on the CR at time of creation and done by WC users. This may or may not be living information, editable at any time during the process. (Though I would shy away from having this be editable and instead just be static once the CR is submitted for consideration.)
Second, the living document that can change throughout the process and be done outside of Windchill, yet affect the CR/be accessible within the CR. Seems like that "outside Windchill" bit is going to be your big hurdle. I mean, some software will have to communicate with WC so the users will have to learn that GUI instead. I would guess you're looking for something kind of very simple interface for this, right?
Interesting idea. Please be sure to update the thread if you get a positive answer from tech support.