My dongle has one button to enter DFU mode that I can press comfortably whilst plugging in the device to the PC USB port (I also have USB extension leads so my PS3 USB socket is right next to my PC USB socket on my desk).
Also, my makefile is set up so I can just type "make dfu" from my source tree and it rebuilds the payload from source and reflashes the chip. Generally, if I've been fiddling with payloads already, that's just "up and enter". The actual flash process takes less than a second, but even doing a full build takes less than 5 seconds on my machine.
You'll find the biggest problem is that the PS3 doesn't supply power to the USB bus until just before it checks for the dongle presence. So really you'd need to have button down = image 1, button up = image 2 and be holding the button from the moment of power-cycling the PS3 until the dongle was detected for the "other" image.
BTW, I'm not saying I don't see the point, I'm just saying that practically it'd almost certainly make the dongle less easy to use. Also, if you need to swap between PS3s that often, surely it's easier to just get another dongle?
That sounds like a badly designed dongle then...