Here’s one to check. I had a similar leak coming from my MD30 gearbox. Not sure if it’s the same config…. There is a small plate on the top of the gearbox secured by 4 allen bolts. From memory the gear selector post passes through it. That plate is sealed to the top of the gearbox casting with liquid gasket. In my case (on purchasing the boat from the previous owner) that was leaking, and was fixed by application of some fresh liquid gasket (Loctite, or Volvo, for 10x the price) and careful refitting. It took a while to figure out that was the culprit.
The membership association for owners of Dufour Yachts