Mayhem--I don't know much about electronics, but what if on every lead in the cable you wired a diode? I don't know exactly where, but if you did that, then it would prevent the electricity from traveling bck up the mic lead....
it is very possible. sometimes when doing gigs with a smaller board, channel space becomes a big issue. so i soldered my own adapter to run two rack tom mics into one channel. works like a charm every time!
Whilst it might work like a charm, it may still cause damage.
I have/had the schematic for a simple resistornetwork for summing two signals. I also have the schematics for a summing amplifier as well, which is active and therefore does not cause any signal loss, like the resistornetwork will.