---*** HUGE SPOILER ***---
I tried this out just to see if it could be done
... and it actually does.
1) Have a mule character (or least desired character) use the lamp on him/herself.
2) As soon as the genie appears, drop the lamp on the ground.
3) Click on the character who you wish to receive the wish, before genie finishes speaking.
4) When the genie finishes speaking, choose your wish (you may request any wish unlimited times per character).
5) After the genie fades, have the mule character pick up the lamp, and re-use on self.
I did this post patch, and was able to raise party stats (except for lamp-user) to 24, and grant Arm of Argus, and give unlimited wards, armor, swords. It could be used for a slush fund, since the armor & sword sell for 20K plus. If you want to see your level 13 warlock meleeing for 20+ with the wizards dagger, this is certainly a shortcut.