Although I am on vacation through the weekend, I couldn't help but post this half-baked variation on Spell Use.
This is not for UWoM, just an idea:
Level- | Percent Chance to Retain Loaded Spell-
01-04: (2x Intelligence) as %
05-08: (3x Intelligence) "
09-12: (4x Intelligence) "
13-16: (5x Intelligence) "
17-20: (6x Intelligence) "
:: Above percentage is divided by the Spell Level to determine chance to re-cast the spell. Failure results in the standard Vancian 'use' of the powered spell matrix.