Fitting shower boxing onto plastered wall

[mrsplodge]

We're renovating our house and are currently figuring out how to do the bathroom. We took it back to the brick and had it all plastered (plasterboard + skim). Our next step is to sort out the boxing for the shower. It's a wall mounted shower, but unfortunately the brick wall is a very thin, non cavity wall (the bricks are laid on their side and are only a single layer thick). So we can't really chase into the bricks for the pipework, so are going to build a thin boxing layer to run the pipes up. I think we'll be using Hardiebacker for the boxing, and then tanking / tiling on top, but I'm wondering if it's OK to attach the batons + cross pieces for the boxing on top of the plaster (so the screws would go through the wooden batons, through the plaster and into the brickwork), or if I'd need to chase out the plasterwork and mount the batons directly onto the brick? The boxing won't be very big (700mm wide and floor to ceiling - about 2.4m, then only about 3 - 4 cm thick so that the pipes for the shower can run up the brickwork and then have enough room for the right angle to come out for the shower), and will have Hardiebacker 12mm and tiles on. I'm thinking 3 batons, then some cross pieces top, middle and bottom. Also, would I need to attach the cross pieces at the top & bottom to the floor / plasterboard ceiling and walls, or just to the walls? I'm thinking for the bottom ones they can be screwed to the floor only, and the top ones can be screwed to the wall only (as the ceiling is just plasterboard).

I realise now that the proper way to do it would have been to build the boxing first, then get the plaster done up to the boxing, but if we chase out the plaster now to fit the batons to the bricks, we'll need to replaster back up to the boxing to tidy that up and I'd rather avoid that if we can.

Thanks