After reading the linked article I wouldn't necessarily go off the idea of using the donor vehicles ABS system.
It seemed to me that problems are only encountered with the ABS if you change dramatically the brake components. If you retain the original donor's brakes without increasing calliper or disk size it should all work the same.
The one thing you have changed though is the weight of the vehicle.
This I would of thought will have an effect on the ABS, in that the brakes will pull the vehicle up quicker because the car is lighter.
Though I bet the software program is the same for all models of the same car, be it a bog standard 1.1 R25 or a fully kitted 2.0 TD pulling a lot more weight about.
Taking this forward a good analogy in weight of the vehicle as a variable would be when towing a trailer or caravan.
Worst-case scenario therefore could be 1). a car (R200/25) one day being driven solo by a 50kg women and 2). the next day the same car towing a 500kg* caravan and with 5 rugby players in the car itself. Now that must be a weight difference of over 1000kg! This indeed would be more of a weight difference than our 5exi transplant!
*This is just a guess, as I do not know the weight of caravans (but it cannot be that far out) or even the max weight a R200/25 can tow.
Would be very interesting to hear from a braking specialist/boffin.
Mike.N