Or possibly clever counterweighting under the floor, if you have the floor space. Old-school mechanical linkage might not work very well (too complicated to get right, probably), but a few sensors and actuators could do the job very well, or I suppose as much as it offends my computer nature a manual level somewhere could work too. (But soooo much less cool.) I don't think this could solve the entire problem, you'd still have to consider weight distribution, but it would free you up a bit. Trying to do something like this involves enough constraints as it is, anything you can do to relax them will help.