I was talking to a bloke with an old DKW and he said that the correct lubricant for his gearbox was a very thin lightweight grease, I have to take his word for this as these bikes are outside my experience. Point is, is the BSA gearbox a likely candidate for the same treatment? I can see both advantages and disadvantages for this, but more plus points than negative if the box is of a suitable design to handle it. Grease packed gearboxes are common enough in other situations. Over to the mechanical experts!
I recently put a sealed bearing in the gercase and although I had other issues with bike, I went over and back to france with no visible change in oil level - approx 450 miles this year
Grease was not recommended by BSA to my knowledge, so oil is best, keeping it in there is the problem, so grease becomes an easy solution to a gercase removal/bearing replacement
For what it is worth, my advice would be, to bite the bullet, change the bearing and use oils as recommended
Although not completely cured of oil leaks, primary still weeps oil a bit, I can park it now without too many concerns of oil puddles and constant oil level checks