First thing I did in the french house was, of course, the bog. Didn't realise some of the internal walls were in these blocks. Had a sink to fix to the wall. Discovered what you did. so ended up cutting a strip out of the plasterboard then doing similar to you to fix a thick batten to the wall flush with the plasterboard. Skimmed it back in etc, so it looked OK and bolted the sink to it. It is still up 15 years later and I know that W uses it to lever herself up off the loo as it is the only one with no disabled toilet seat in the house! (There is a proper bar on the other wall for her left hand!) Finking about it, that wall is load bearing as there is an upper floor above it, the rest of the house has no upper floor.My first house had the internal skin of the walls and all internal 4 &1/2" walls in those clay monstrosities.
Hard to hang a sink to a wall when the wall is only 1/2" thick effectively and 1/4" of that is the plaster.
My Dad taught me how to resolve the problem by knocking out the "failed" screw mount holes and filling the clay "pot" with mortar and setting studding (with the tails cut in 2 and fan-tailed out) in the right place. A solid fixing was available within a day.
On the plus side...... the internal wall betwixt bath & bog was easy to take down.
But yup, total pain in the rear!!
