@ Sammay and Ken,
right-clicking on white is already taken up in my case - with favorites. I wouldn't want to give up on that, especially since I have Sammay's script to add dual favorites on the fly. And I find it counter-intuitve to right-click white in order to do s.th. with a selection - everywhere else it requires right-clicking on the selection itself, not in the white space.
But that's just me. It might help CookieMonster better than taking the route via "Open with.."
Your approach has many advantages over mine. It's more direct, on the first click. And it allows better organizing with separators and whatever else is not possible in the "Open with..." dialog.
Ultimately, I'd love to see customized mouse actions in XY - it'd be a nice complement to shortcuts. Nothing faster than clicking AND pressing shortcuts, combining modifier keys and acclerators with left/ right clicking.