I was actually mixing 2 different things. One is the balloon dialog, that can be easily disabled in the registry. The other one is XP's monitor detection feature, which disables higher refresh rates when booting with no monitor attached. I think this won't be a problem anymore once I finally attach my PC to the arcade monitor (a normal 15KHz type), but it's quite annoying now that I'm using different monitors and TV sets for testing.
The interesting thing is that QuickRes (or maybe, the modified driver itself, as this also happens in the standard XP resolution selection window) gets fooled by Windows into thinking that refresh rates higher than 60Hz are available: if I start the PC without a proper VGA DDC monitor, I can select those modes but I only ever get 60Hz instead. Maybe the author can make the driver aware of this and gray out those modes.
This behavior also shows up if the monitor is turned off at boot time.
At least this is my understanding of the "problem". I'll update this thread if I find out more.