Like Senso, I am not sure that I have understood leofls.
But, it is feasible to have the bootloader look for something to shorten the bootloader delay. My question is what?
If it is EEPROM, then existing programs which use EEPROM might break.
It can't be the Reset button as that is a physical piece of hardware that forces the microcontroller to reset and not run any code.
If it were a different button, then an I/O pin would be lost (and I run out of pins already).
So, leolfs, if you want a shorter bootloader delay, you could blow a new bootloader into your chip, and that would work. I believe Ladyada has a quick-boot bootloader, so you could start there.
But I can't think of any other hardware way you could do it without changing the board design, is a way which some folks (including me) might regard as a dis-improvement.
You might try to change the bootloader, so that it has a short delay, and maybe change avrdude (the upload program) so that it can force a reset back into the bootloader? All this stuff is Open Source, so you could do it.
[edit]What is the use case you are looking to fix?[/edit]
HTH
GB