I don't think much can generate heat like that. If it was a bearing seized, I think you would get vibration, especially at the propshaft end. Conventional (classic) gearboxes usually run in top gear by connecting first motion (input) shaft directly to the third motion (output) shaft, so the lay-gear and gears are freewheeling.
Way back in time I had a 1275 Midget with a really badly worn gearbox. Much noisier and looser than was good for it, but I just used heavy duty oil and molyslip in it because I couldn't afford a repair.
That gearbox also got really hot on the M way. So hot that the gearstick was too hot to hold, and it tended to stick in4th, needing a sharp swipe to come out of gear. But it never let me down.
So I would check the feel of the propshaft where it enters the rear of the gearbox. If it seems OK, just see how it goes. Maybe do another oil change and see what comes out? Assuming you have a drain plug!
