not sure how others do it, but at the moment i reserve one of my default UI bars (the bottom right one) for cross team spells.
Basically the whole bar is repeated on all toons and each of the dozen available buttons has a emergency/special ability of one of the slaves (grounding, earthie, B-Rez healer, treants, innervate healer, iceblock, poison cleansing totem, etc

All i do is put the corresponding spell on the bar for the specific toon, and leave other buttons empty.
and on the master i put an empty macro (just the correct icon) to remember what spell is where.

Then in game, all i have to do is click my earth elemental button on my paladin and my shaman casts his earth elemental

As Ualaa said, all it lacks is the cooldown, but since i mainly use emergency spells, it's pretty obvious when they are available or not.