My buddy says engines suck real good so no worry about gravity feeding them. But for the AC system, I will have to do as you suggest, put the bucket on a ladder or roof of car.
Currently I remove 4 hoses (2 engines, genny, AC) and it's a royal pain. This will save me time, aggravation and cut knuckles (yeah right).
And for the record, I tried back filling AC but it doesn't work. I have two units and one pump so there must be a check valve in there somewhere.