Unfortunately that idea was a failure for me, after 1/2 day riding on wet roads on Sunday, the gasket sealant failed, I inspected today and the valve was loose and easily moved inwards again. I have seen many photos on various threads on other forums of missing valves or pieces of valves visible inside the drilled holes where they fell in. It's difficult to imagine any valve fitted per the BMW retrofit method remaining in place for very long on any bike regularly ridden on normal roads.
My latest attempt is zip-tieing the valve to a short piece of 10mm hard plastic tubing which is then inserted into the hole, It's a much tighter fit but even at that I wouldn't be super confident it will stay in place, but let's see how it fairs.
I did pull back the rubber boot on the drive joint for a brief inspection of what I could see of the driveshaft and knuckle joint, It looked pretty OK, but I believe now after reading other threads that the best thing to do is remove the rear wheel drop the final drive and lube the driveshaft splines and then do this regularly at 12K intervals. The drain valve or even just the hole without a drain valve is probably worth having to keep any large quantities of water from accumulating in the housing and accelerating corrosion, but a regular visual check and lubing of splines seems to be what many have decided is the best preventative maintenance solution.