Ok, but did you try to install software in there? I know you used the demo of Crossover since you were evaluating, I was more wondering what software you tried to use with Crossover.
In any case, I have managed to sort of reproduce your problem. In my opinion, this is a bug with the GUI. On my system, just removing z doesn't seem to change much as the bottle manager's interface, and the bottles themselves, just work as usual. I did find that removing the y drive (to my home directory) did finally kill the control panel with everything in it gone. Winecfg could still be accessed by using the "run command" button in the bottle manager's GUI. The software in the bottles still worked as before, even with only the fake c drive and a faulty control panel. So, since the software still runs, I do think this is just a bug or a bad assumption made with the bottle manager that expects both the c drive and a drive towards the user's home. That means you could easily remove every drive except the fake c drive even then, it would just lead to an empty control panel in the bottle manager.
That being said, I can't but remark that this is the first anyone tried removing all access. If you're worried about malware, at this time, basically nothing works. Wine / Crossover do not offer the entirety of Windows API, and certainly very little that is low level. That means little in the way of malware actually works, and I never heard of a serious security problem while using a windows software under Crossover. Further, Crossover doesn't circumvent normal file permissions. Access to / doesn't mean root privileges won't be necessary, and Crossover doesn't even have a mechanism to get root (or sudo). So unless you run windows software with Crossover under the root account (which you shouldn't), there are very little concerns to have. What I'm saying that, although I understand your concerns, I'm not convinced they're justified. If you are that concerned, you have to consider that all software on your system has some read access to / , and I don't see why that's any better without going through Crossover.
I'll report the situation to the staff later and see their answer, but you could do the same by writing to email@example.com. Since your installation seems more fragile than mine (I can remove z drive without problems), it may even be more interesting if you did so.