Why not go Dan's idea a bit farther - and park one wheel at a time on the scale? Again, as Dan mentioned, the totals might not add up to precisely the total weight of all four at once, allowing for both inherent scale repeatability issues as well as weight transfer not allowed because of a stiff chassis, but it should give you a close approximation. Try, of course, to find a scale where the approach is flat - not a scale that's up on a platform, for instance -- so you don't get results skewed by the goofy angle at which the car has to be parked.
