I’ve made good progress on the MuxShield – only some minor changes to the bootstrap were needed, here is a diff:

These extensions still fit in the current 2K boot loader area, but only just!

To summarize: the bootstrap watches the switch connected to pin 7. If it is high (which is also the case if the MuxShield is not connected), then nothing changes. But if it’s low, the following happens:

• Change the bootstrap baudrate back to 19200.
• Respond as if this is an ATmega168 (different signature bytes).
• Store all incoming flash data at an offset of 0x3800.

The end effect is really, really simple: if the switch is in the RUN position, nothing changes and the Arduino behaves just like an ordinary ATmega328 board. If the switch is in the PROG position, the board looks like an ATmega168 board while programming. After that, it resumes its ATmega328 program as before.

One more step is needed to make this truly plug-and-play for re-flashing up to 5 connected FTDI slave boards: on startup, the MuxShield code must check if the switch is in the PROG position. If so, it must program each of the slaves with the data loaded in flash memory starting at offset 0x3800. Then finish off by starting the serial multiplexing logic.

Anyway – 2 out of 3 steps are done, this is going a lot smoother than expected!