The "four discrete button" approach is a good one, but the fact that two of them are associated with a separate control that's being intentially appropriated (with good intentions) by the other two does put a heck of a wrinkle in the issue. User interface design decisions invariably produce compromises of some sort - and just as invariably, that fact of life offers little comfort when you end up on the wrong side of one of those design decisions.

I haven't tried this (rarely use the second zone of my 990), but I wonder if there's a command that would serve to "cancel" the second zone's control over the front panel display prior to the five second time-out? I can't think of such a command (maybe hitting the 990's MENU button twice?) but it might reduce the hassle factor for your situation...