OK will do :)
But even though the fortresses themselves don't generate, the *location* of fortresses still work. I.e. in the original (single player) skyblock I build my own nether fortress and my own witch hut to farm those, and since they were in the right place, the withers, blazes and witches spawned there because that's what the location was.
It was easier to find the witch hut and nether fortress locations because I had the world key ;) and I could spawn the regular world with that key and find them. But I might be able to find a nether fortress *location* by building a gigantic matrix of grids. Although I probably wouldn't ;)
But I will look for you & see about your wither grinder! That would be awesome and much appreciated!