As I see it, and totally untested of course...
Let's assume 5 processors in total.
All resets tied together.
All SCKs go to one side of a 2x5 header.
All SCKs pulled high with individual resistors.
Before programming pull all but one jumper.
When done put them back.
Could get a little tedious and there might be a better way (say with some logic, a 4-way analogue switch chip or something) but that's my first take on it.
All the other devices still have MOSI and MISO connected, so I assume since their SCK isnt connected, they dont sync to the data coming in, so they ignore it?
I also assume they dont try and send anything during the programming saying 'I cant understand!' etc... which could cause problems?
The others can't do anything, they are in a reset state. (assuming they ignore any MOSI data because they don't have SCK, a reasonable assumption I think but far from proven)
If the device being programmed is disconnected from SCK, then all the rest could still talk...
No, reset remember.
And a common ICSP connector between the 5 devices? So disconnecting the SCK from 4 of the devices will leave the ICSP connectors SCK being connected to the device being programmed.
I'm reasonably certain it will work and so far nobody has thought otherwise, but I wouldn't get 50 boards made