One of my players abs proposed a feat that essentially works like the daily power on the Staff of Missile Mastery (which generates multiple missiles during one power activation). Is there a similar feat/exploit/class feature already in existence that has a similar effect?
Take a look at Wizard's FuryDDI from Dragon Magazine #374. It is a first level daily power that allows the caster to use Magic Missile as a minor action 1/turn until the end of the encounter. This would allow the character to cast Magic Missile twice a turn for one encounter.
Allowing a character to essentially double the damage from an at-will attack more than once/day would be a very powerful effect. I would only consider it as part of an Epic Destiny as a general, always-on ability.
With respect to maintaining game balance while allowing such an ability, there is a divine feat Light of OrderDDI from Dragon #381 that allows the character to swap out a 2nd level or higher utility for an encounter power that is triggered on a hit and adds 1d6 per tier damange to one attack.
Light of Order will average an extra 3.5 damage per tier per encounter and unlike the extra magic missile from the staff of missile mastery it is single target damage, which tends to be more valuable.
If you imagine a feat that allows power-swapping in the same power as the daily on staff of missile mastery, in my mind you could bump the power from daily to encounter without harming balance.