Good luck. ZBrush is probably our number 1 requested application. I had a meeting with the Pres and VP at SIGGRAPH a couple of years ago, but they proved to be 100% unresponsive afterward. It is a VERY cool application...but it really needs to have 3D mouse support.
You are correct in diagnosing the problems you will encounter. Even their "SDK" may not work if it isn't designed to handle continuous navigation in the background (while other code and other plugins are running). It is all too common for application code to assume the view will not change while it is in the middle of doing something...then blow up when its assumptions are broken.
We've been pushing on them, but frankly the only people they listen to are paying customers...