How can I shorten barstools temporarily so I can reattach the bottom of the legs?
This sounds like a problem that's best avoided rather than solved.

I'm picturing a "classic" wooden bar stool where the legs are not vertical, but tapered out to be wider at the bottom than the top, e.g.:



This means that the legs experience a bending load when someone sits on the stool (imagine someone trying to pull the legs wider apart at the bottom).

The parts of the legs that help resist this bending load are the wood fibers around the outside (circumference) of the leg, the center of the leg doesn't do much (and doesn't need to).

If you cut the leg and re-attach with a hanger bolt or double-ended lag bolt, you've prevented the critical parts of the leg from doing their job--you've severed the outside fibers, and attached the leg only in the center. You can imagine that a leg put back together in this way wouldn't be hard to break over your knee. Therefore, it wouldn't be great in resisting the bending load that it needs to. It'd be fine as a plant stand, but I'd be sweating if Norm stopped by for a drink.

Norm http://barsbyal.com/user/cimage/c-norm-03.jpg

The only solution I can think of to retain the bending strength is some kind of a slip-fitting like you'd find on a patio umbrella or kayak paddle. By the time you're done buying and installing all the slip fittings, you'd probably be better off buying a second set of stools.