Tee slingshot sure would be fun .
But it would help mostly when fighting against a team I think, or rather when trying to get away from it with the flag. In those situation the double hook situation happen spontaniously as you hook to get away and they hook at you to stop you. As it is now the odds are really against the runner when catched like that, the slingshot effect would shift the balance somewhat since the defenders would need to kill fast or the runner would fly away on hook release . I don't know if that would be good or bad for balance, but it sure would be fun.
As for dummy dragging, I imagine it could be balanced in a way so that only gets harder, not impossible. Might even be possible to slingshot the dummy around some difficult corner .