David Kiehna will know for sure, but I think I recall him saying it wouldn't quite fit. I think the Miata headrests are a little taller.
Wait a second, I'll go out in the garage and check....
OK, this isn't exactly scientific (after all, I AM on my 4th beer!) but, sticking a 1X6 behind both seats down to the floor, my '95 Miata seats appear to be about 1.5"-2" taller than the '78 MGB from the floor to the top of the headrests.
Now, back to the Warsteiner....