I wonder if a Merit would do it.
It probably wouldnt be worth the trouble for a vanilla mortal, because the thing about classic CMAs is that they were generally too power hungry for a vanilla mortal to run, especially the post-Form effects.
That seems acceptable.
Combat turn is 3 seconds. Just about the normal duration to activate a shintai.
Point of correction: DBs can and did use CMAs; pre-Usurpation they were all they used to expand outside their native skillset. The Elemental Styles were invented post-Usurpation to give them a leg up against hostile spirits that didnt recognize their overthrow of the old order and rogue Anathema.
Nothing prevents DBs learning the charms of Violet Bier, or Crane , or even Solar Hero Style.
While Five Dragon Style is a solid TMA?
You'd be thematically better off looking at the styles designed to give mortals a leg up against supernatural threats.
Like Janissary(anti-CoD), or Ivory Pestle(anti-ghost).
Golden Janissary already draws inspiration from the Sun, which in Dresdenverse disrupts a lot of magic enchantments, and weakens or kills creatures of the night. Bringing a version of that back to a world where Red and Black Court vampires burn in sunlight? Yeah.
Assuming Mavra et al get frisky, reverse-engineering that and just proliferating it among the multitude of lower level mortal organizations might have interesting effects.
Something to keep in mind.
