Just an update : I found someone who knows about powershift torque converter gearboxes, it seems that they cannot be towed either. The pump which lubricates the gearbox is driven off the engine, so whilst you'll get away with towing a short distance 7 miles would wreck the box. Back to square 1. Now looking for a 1:1 drop box that you can put out of gear. I've tried marine, they don't tend to have 2 output shafts. The other option is a bypass and electric boost pump on the existing hydrostatic, but we'll out of my comfort zone.