[paulgato]

OK, I may have misread something. You could, I guess, 'T' off from the feed to the heater matrix, and fit an ordinary wax thermostat (grab a suitable housing from some car or other). Choose a thermostat with a rating of say 80 degrees C, so engine will heat up to a reasonable temp first but heat will go to gearbox before going to radiator.

But I still reckon you should first exhaust the possibilities of reducing friction by using the best, low-temperature-friendly gear oil you can find. That is SO much cheaper and easier, and my experience of doing that was very positive.

(The other possibility, which to my mind may be easier to implement, if a little less efficient in absolute terms, is to fit a 12v electric heater pad to the gearbox. The advantage of that (apart from ease of installation) is that it would start warming the gearbox as soon as you switch the engine on, and would not delay engine warm-up. It would be easy to fit it with a voltage-sensitive switch so it comes on only when alternator is running (or if you have a mains charger plugged into the car at night? Maybe then fit a similar heater pad on engine block as well?) Also maybe fit a thermostat on the gearbox housing so it switches off at a given gearbox temperature. Generating electricity with an alternator is not that efficient, but the losses mostly end up as heat in the engine bay, which is kind of what you want, so maybe not that inefficient overall.)