I haven't tried making V4 JCMs yet, but certainly for V3 it was common practice to have separate "up" and "down" collar JCMs, which were switched by setting limits. There are a couple of threads at PoserPros which may be helpful, although it sounds as if you have it more or less licked (see how I introduced a cat pun there, LOL).

How to make matching JCM morphs in your clothing, a Tutorial
I made a collar JCM :-) - and, a suggestion...