The idea has been floated before
You wouldn't need to do anything to the front diff - the rear pto is powered via the outer casing of the diff
Thats the front diff sitting on the transfer box (its an 02M diff/transfer box, but same design) - the rear pto drive splines are below the roller bearing - the splined shaft sitting inside the diff is the right hand driveshaft pto
The transfer box has no step-down gearing, the prop runs at 1:1 ratio
iirc the rear axle diff casing can be detached from the central haldex diff unit
The rear diff is standard type of thing, easy enough to weld up
all driver aids can be disabled via vagcom
iirc the last chap who thought about went and bought a bmw ..