Hello Norm, I am slower on the uptake than others and been left behind by the way this discussion has progressed, but years ago I was an inorganic chemist and actually worked with superacids, mostly fluorosulfuric but also some of the mixtures (stronger acids) with antimony pentafluoride. Basic answer to your question is that the strongest of these acids will protonate (add proton to make a positive ion, so copper metal reacts to form Cu++ and soluble in the acid, for example). Same happens to almost anything so you would generate a lot of ions in solution and a lot gases (hydrogen and other gases depending on the nature of the acid that are produced by the disolutions) that will boil off making frothing etc like you suggest. Separating out all those ions would take engineering solutions that are beyond current day technology (chemists can do it on a small scale, but not an industrial one, at least not cost effectively).
One difficulty you would have is that all superacids react with water so your vat could not be in our atmosphere because the acid would react with the moisture in the air. They would also react with metal containers or glass containers (not all of them, fluorosulfuric acid and triflic acid are stable in glass but the stronger superacids would not be). Some fluorocarbon plastics (teflon for example) can be used for containers.
On a different level aren't you asking an old alchemist question, the one about alkahest, the universal solvent. If that's where you're going I would suggest a superacid that protonates just about anything is as good a bet as anything else.
Hope this is some use.
