i've got a bike (a customer's bike) with old (2003-ish) 4 piston shimano deore XT (brm-755) hydraulic brakeset that has a bad--leaky around the pistons--caliper, and there's no way to rebuild it. the shimano people and quality catalog urge me to believe that the lever cannot be paired with with newer model calipers. anybody have experience pairing *supposedly* incompatible levers with other calipers? please advise.

You can't pair them up with XT levers that has the servo wave feature. Too much pressure for the seals. You can use them calipers with any other levers as long as it does not have the servo wave.
I have them with XTR levers and they works just great. Looks bad ass too on the bike