a total hack job, but better than nothing... and certainly better than an NSBeep()!
for anyone interested, this is what i ended up doing to more-or-less solve this problem:
A- set another (secondary) "Quit Application" menu item under the real one (but i couldn't use the key equivalent Apple-Q because it's taken already)
B- wrote a new method that first ends the sheet and then kills the app.
C- make a hidden button on the sheet that is equivelent to Apple-Q, set to the method mentioned in step B.
D- wrote outlets to the real Quit App menu item and the new secondary Quit App menu item
E- set the menu items hidden YES/NO according to the situation: on beginSheet method, the real Quit App menu item is hidden, and the secondary one is visible. on endSheet the inverse.
there are 3 problems with this:
1. when a sheet is visible, quitting from the dock menu will not work.
2. the secondary Quit App menu item doesn't display Apple-Q as it's key equivalent, as that key equivalent has already been taken by the original Quit App menu item.
3. it's ghetto!
something so simple as this that takes so much effort to implement is definitely something that needs review from Apple.