I plan to build my shower bench using mud with masonary blocks and bricks. I have a PVC presloped liner installed and will use mud to set the bench on the liner. I read that mud will not stick much to the Hardibacker walls, but the mud will add to the stability of the bench.

Just checking to see if there is a problem with building a shower bench with Redgard already on the Hardibacker walls?

Use modified thin set to bond the mud to the Redgard. As you build the bench, make sure you have wet thin set on the wall. When the mud hits the wet thin set, it will bond to the mud, and the thin set will bond to the Redgard. Just trowel it on the wall, just as if you were tiling.