WBP ply or backer board?

[HighlandWoman]

Hello, long time lurker and kinda first time poster looking for advice on bathroom planning.

 

TL;DR: Will tile backer board fixed over brickwork hold grab rails or is WBP better?

 

My bathroom currently has been lined with plywood and then tiled. I am looking to remove the tiles and reconfigure so I have a larger shower and a Japanese style sit bath. The shower and bath will run along an internal party wall which might be brick (assuming it is the same as in the kitchen. Previously the original lime plaster was chopped out and at least half the wall replaced with ply and then tiled over.  The other end of the wall looks like it might be plasterboard.

 

As this wall is where the shower (with concealed piping) and bath will be, it is going to be tiled to about a foot above the shower head (ceilings are 3.4m) and the shower opening will be at one end of the bathroom, with the bath accessed via steps built into the other end of the shower. These steps will create a shower seat as well. I want to attach a couple of really strong grab rails at the transition between shower and bath. I'm not convinced that tile backing boards are up to the job of holding a grab rail and I'm not sure where the rail will go to put studs in place. I can drill all the way through to the brickwork, but if I hit mortar, then I'm not convinced that is going to be safe.

 

Whatever I use, I plan on completely tanking the shower/bath area.  I have attached a rough plan but it's not to scale and I struggle toggling this kind of stuff. For the record the bath is 100cm high and I have checked I can get a Gerberit wall cistern in there which will service a washlet.

[Temp]

Not sure about grab rails but tile and stone over 12mm Hardiebacker board seems to hold things like soap dishes and shower head rails well without needing to find studs.

 

I think 12mm of anything might be marginal for grab rails? Can you drill through into the brick?