Aha! I think I've got it.
The character's Multipower has Variable Lim (Gestures, Incantations, OAF, Extra Time, and Costs END). Everything is normally on Charges, and doesn't cost any END, and none of the slots have the Costs END Lim.
3 Silent Spell: Endurance Reserve (10 END, 10 REC); REC Recovers Once per Day (-4), Only for Replacing Incantations with Costs END (-0).
Now, he can cast 10 END worth of spells silently (the largest spell right now costs 7 END). If I wanted to further limit it to 1 spell, I could throw on a 1 Charge Lim on the Reserve.