Anyone tried it?
Drain out all the coolant you can, remove the hose from the coolant pump discharge, put the shop vac hose on the pump discharge (you'd probably have to jerry-rig some sort of adapter to get good suction on the coolant system), and put the coolant hose into distilled water, letting the water be pulled through the system by the suction of the shop vac.
Once enough water has been pulled through the cooling system, let the vacuum run for 20 minutes or so to pull as much water out as possible and dry things out, so the new coolant you put in isn't unduly diluted.
Would this flush the system sufficiently? Would the suction cause any problems?