Obryn
Hero
I think a better question is whether or not a short rest is compulsory. If one PC takes it, does everyone else have to?It may just be the way my games are played/run but how often do you really have two separate encounters without a short rest in between?
Also the only way temp hp stacks is with a special fighter build, and they don't even stack that much.
For Kol Korran's boon, lets say everyone has taken an Extended Rest. The 6th level Cleric uses two heals, and he gives two characters Surge+2d6+5 temporary HPs. He takes a short rest while those two work out or explore half-heartedly or something. He does it to two more party members, takes another short rest and hits himself with it. Maybe he re-hits the Fighter if he rolled low, but it's probably not worth it.
Now the party is fresh, and each one probably has over 20 temporary HPs going into their first battle. This represents a significant percentage of their HPs - over 25%, and possibly up to 50% - at the cost of a single healing surge each. (And an extra 15-20 minutes, which is hardly ever a big deal if they just spent 8 hours resting.)
Don't get me wrong - I would nerf the hell out of this in the unlikely event a PC ever tries it. But it's far from impossible.
-O