Did a little searchin...
Apparently, the later (post-renix) Police-issue XJ's had an "Extended Idle" switch that'd bump the idle about a grand.
Kicker is, the PCMs are all the same, so the circuitry is there. Find the right lead, ground it through a switch on the dash, and bingo.
http://www.naxja.org/forum/showthread.php?t=35197
Post #21 is where it starts getting good...
Robert