So, my Elite got fried by my disconnecting the battery without double-checking to note that the ignition was in the on position.

I had to purchase another-don't want to do it again and can easily see it happening again if a mechanic working on the car isn't careful.

Is there any sort of protection circuit I can build/buy that can protect against this?

It needs to protect against power energized, but no ground.

 

Haltech kind of covers this in their battery relocation video.