I've never done it on a 750 Sei but you should be able to change the gearshift oil seal with the shaft in place. I will be hard though. There are two methods I heard of but I've only used one.
1) Take a fine drill and drill two small holes to suit self tapping screws through the metal part of the oil seal. This is a top hat shaped ring that gives the seal its strength. Screw in the self tappers and yank on them. It's quite easy to make an improvised puller arrangement with the self tappers.
2) I have seen very thin oil seal pullers for slide hammers that you force past the seal lip and hook behind the metal top hat. Never seen one for sale though!
I think the the oil pump, clutch pushrod, neutral switch, and output shaft seals sit behind that cover as well. Any of them could be leaking.
Hope this helps