Spellcast Help With Migawari

I've been trying to write a spellcast rule that locks Ningi+2 body when Migawari is active, and haven't been able to find a way to do so.

Basically I want it to look like this:

This won't work, because Spellcast doesn't recognize Migawari as a buff. I can't find a spellcast BuffActive that coincides with Migawari either. Am I missing it, or is there another way I can accomplish my goal? Thx..