I'm not recommending this, just telling you what I did once over.
I had a major oil/water leak (the water got in, and lots of emulsion came out on a run down the M6 enroute to Cornwall) from the right-angle drive box on the front of the shaft on a Suzuki 850 GS. It only happened at prolonged motorway speeds. A friend who sold Tractors for a living recommended some stuff used to seal leaks in tractor hubs. It works its way in to the leak, then sets like a gel, sealing it. Never had a problem after putting it in.
No idea what it's called, but an agricultural supplier should be able to help. Thoughg myself I'd be replacing the seal again, and maybe having a look at the input shaft while I was at it...