Completely Understand- the thing is though, wooden batons are more than acceptable for this application and are usually very effective. The main problem it seems is the bath simply hasn't been installed properly and it's leaking past the compromised wall seal, that's what needs to be fixed. There is no other way that the wood beneath is getting damp and mouldy unless there is another leak under there, say from the taps/feeds or the waste.
The long and short of it is that id nothing else is leaking then it won't really matter what material the supports are made of, if they get wet again then the bath can't be sealed properly. If that happens it won't really matter what the supports are made of as the wall and floor (and poss ceiling below) will eventually be water damaged regardless, if you see what I mean.
The key is the bath needs to be installed properly with the batons fixed to the wall properly and then the bath fitted to the batons properly. Then the bath sealed onto the batons and the wall properly. If that is done properly then there shouldn't be any more problems with the wall seal and the wood beneath should never get wet or mouldy again.