Aerobatics in "firm" IMC -- how would one do it?
No - you don't do it.

First and obvious reason, there's no body to see it.

Perhaps much more important is safety though. When pulling a loop, it's not like flight simulator where you just yank the stick aft and watch the attitude indicator (AI). An aerobatic pilot is constantly checking his attitude by looking at his wing tips, quickly glancing the left and right sides back and forth.

In fact, the AI is usually caged (meaning it is locked to prevent rotation) in such violent maneuver. The proper procedure is cage the AI, perform the maneuver, check straight and level (visually), then uncage the AI. If you don't, the gyroscope will tumble quite some time before it settles again, rendering it useless. Meaning, if you perform in IMC, you won't have accurate AI information for recovery.

For nearly vertical maneuvers (e.g. stall turn, rolling while vertical), even a functioning AI wouldn't help, since horizontal direction is undefined in this state. The AI and compass will just flip, yet you have no way to determine which way you go.

Violent G-forces also mean it is much more difficult to read instruments during such maneuver.