Well there ya go. I should have done my homework, I was working from my feeble memory.
The SVX, that we in the states drive, really doesn't have a center diff. The trans is basically a fwd unit and the front/rear connecting clutch allows the rear drive to share some of the work. That is, the front, as long as nothing is broken, always has full power applied to it, and the rear is given some of the load as the TCU sees fit. So if the TCU decides the rear should do 20%, the front, even though nothing has been taken away from it, is effectively handling 20% less, or only 80%, of the total load. Hopefully you can now understand why there is no way to 'trick' the TCU into applying more than equal power to the rear, as there is no mechanical provision to disconnect power to the front diff.
Another interesting AWD vehicle: the 1966 ('67?) Hurst Hairy Olds. A Gutlass 442 with front and rear blown engines, TH425 transmissions (Toronado/Eldorado/GMC motorhome) and no mechanical connection between them, save throttle linkage. The trick was to keep two engines and trannies power sync'd from a dead stop. It didn't work well. If my memory serves me well, the last run the car made was in Gainesville and the driver bailed out the window at speed, with the car continuing on pilotless, blazing a new trail into the nearby swamp.