Stainless steel is an alternative and likely competitive with the coated stuff. I used the galvanised expanded metal squares (from the likes of Th*rne) as they imparted rigidity and accurate shape to what can be lighter-weight wooden parts, but have just made a batch of floors with S/S mesh.
Galvanised wire screens are also available at very competitive rates . I have made travel screens with this, but would not be bothered if it were in my floors. They are all available from the mesh company at competitive rates. I avoided the epoxy as it is not flame proof (as well as not being available at the time).
No connection with the supplier - just a satisfied customer.
Oxalic acid is not regarded as one of the 'strong' acids and is used for cleaning brasses and similar. Not sure of the effect on plain steel, but not much worse than rusting away slowly. Galvanised may be more resistant but not tested. You are only talking here of the same strength as vinegar, for instance, if trickling.
Regards, RAB