Hi Erik! I have been using "in-circuit" programming very often. But I use ATMEL products and programming is easy through serial interface.
I am using only one "hardware switch" which disables all externall devices and resets the micro (also enters the programming mode). It's been made using small ATMEL's microcontroller with UART. This additional programming device can be using like built-in or stand alone.
To program AT89 in-circuit you need 5 wires: RESET, MISO, MOSI, SCK & ground. Programming routine needs also very good shapes of input signals (like SCK). My "programmer" takes care about signals, receiving - sending - buffering datas, it verifies the memory content, you can connect it to PC using only 2 wires (RS232 interface)+ ground. It is very usefull in situation like yours.

Regards,
Jacek