I do hope that I need another MCU at the recieving end to handle the communication and the programming part of the target MCU.
Really, I take that you mean "I do not hope...."

I do know that with the ISP facility all it needs is a Laptop in the site to download a modified code - but then the customer has heard this somewhere and wants it NOW.

It should be fairly easy. If you use the techniques described in NoTouch, the only difference will be that the RS-232 line has a pair of modems in it.

I do not recall if FlashMagic has the means of forcing a low baud rate (if the automatics do not automatically handle it when seeing the modem slowdown) if that feature is not there, send Andy Ayre an e-mail about your need.

Erik