As far as I know, no. The only way you can get a level 50 copy of Moxxi's Bad Touch "legit" without outside assistance is to donate to her for the first time when you're level 50. It's totally worth it, even if it means screwing yourself out of both that gun and the Good Touch for most of the second playthrough.
Do not despair though. You can rely on outside assistance.
Another "legit" way to get your character a level 50 Bad Touch is to level some other character of yours to level 50, then use the bank system to transfer it over to your "main" guy.
A less "legit" way to get your character a level 50 Bad Touch is to use item duplication with somebody who does have it. Duplication is simple:
- You, on the receiving end, host the game.
- Your friend, on the giving end, joins your game. Using windowed mode makes this easier.
- Your friend touches a save point. In the save file the item is in his inventory.
- Your friend drops the item to be duplicated.
- Your friend quickly and forcefully closes the game (if you're using windowed mode click on the × button on the titlebar.) This discards unsaved changes.
- You grab the item.
- Your friends relaunches the game and reloads the latest save file, where the item is still in his inventory.