See Challenge # 121. Ripping off names from DnD. I was originally going to create a mechanic for auras that could be sacrificed when combat was declared to allow you to cast an instant of a specific mana cost from your hand for free, but that not only has very limited design space, but is far more blue than white.

I think I'd prefer just "Whenever ~ deals combat damage to a creature, tap that creature and it doesn't untap during its controller's next untap step." That's the "stun" ability from Kamigawa (Matsu-Tribe Decoy and friends), which does indeed fit mainly in blue and slightly in white (Kor Hookmaster) under the current colour pie.

I think your original idea might have worked, but was indeed overly limited (It's something I'd expect a cycle out of at most)