I had a similar thought before, which is that the bootloader could be modified to look at another digital pin, perhaps digital pin 2. If the pin were LOW, it could do its usual thing of looking for new firmware being uploaded. If it were HIGH, it would immediately start executing whatever sketch was already in flash memory. Then the hardware could be modified to have either a pushbutton or a slider switch to select between these two modes. The bad thing is that this would tie up an entire digital pin, unless the same switch/button could be overloaded to do something useful after the program started running. But this would be ideal if you wanted to make a permanent version of a design, especially if you (a) wanted it to reboot very quickly, or (b) it had to talk to something else via the serial port without getting confused by the bootloader.