Actually an AAD would work fine in many cases for aerobatic aircraft. It's not just rate of descent; it's rate of descent at the triggering altitude, and that's established by setting the Cypress or other unit before takeoff.
When doing aerobatics, you're generally doing them not far from your departure field, just as you're often skydiving nearby. No difference.
Altitude changes in most aerobatic maneuvering aren't that drastic. You're seldom seeing more than a couple thousand feet in the most aggressive maneuvering, and frequently much less than that. Protracted spins lose more altitude, but most of your basic "in the box" maneuvering doesn't consume a great deal of altitude, and unless you're on a low-altitude waiver (which in most cases is below the trigger altitude for even the Expert Cypress), it's not an issue for you. If you're doing practice aerobatics in a 4000' minimum box, then you're well above the trigger altitudes for the Cypress.
One need not static line deploy from an aerobatic aircraft; there are plenty of other ways to go; the most simple of which is get clear, get stable and pull. If unable, get clear and pull and hope you don't horseshoe or wrap. Barring that, even in the event of unconsciousness, one can always go with the AAD. It works.