I had to remove a section of my shower wall for plumbing repairs. The wall is currently drywall. I want to fill the hole with Fiberock. I was unable to remove the tiles from the drywall, so I had to cut the tiles out through the grout. As a result, I don't have enough overhang to butt the Fiberock against the drywall, tape the seam, apply thinset, then Redguard. Is there any way I can seal the seam between the tile and Fiberock without seam tape? Thanks.
How high off the floor is that seam? If above the heavy "splash level" (maybe 4ft) you're likely ok without tape. Otherwise, are you willing to cut a bit higher?
You could also chisel out a groove between drywall and tile, perhaps even just 1/8in thick and reaching up only 1/2in behind the tile, so that you can insert flat flashing behind the grout line at the new interface.
Or you can install "Z" flashing behind the old drywall and in front of the Fiberock.
The real risk here is that pressing against the tile wall will cause cracking of the grout at the seam, forming a path for water dripping down the tiles to enter the wall space at the crack. With the flashing you have at least some protection by lapping.