A good short term bandaid might be to add some DOT 3 brakefluid to the power steering fluid, just a capful or so. The seals in your rack are composed of natural rubber, and DOT 3 brake fluid contains a large amount of alcohol, which causes rubber to swell (reason why European cars do not use DOT 3 in their brakes, only DOT 4, which does not have the high level of alcohol). This might stop the leak for the short term, but be warned, when the effect wears off the leakage will most likely be a bit more severe than when you started, as the seals will then be softer. I can't say this will absolutely work, but if your only other option is getting a new rack, I don't see how it can hurt.

Anyone have any idea how hard it is to put a rack in the 850? I've done them on some other cars, but never a Volvo, so I can't say. Hardest parts are usually seperating the rack/tie rods from the steering knuckle and the rack to the steering column clamp or nut. Requires a good set of pullers and pickle forks. Regardless, if you take it on yourself, have an alignment done right after you put the rack in, they will then center up your steering wheel and all that at the same time.