Don't know molarity, but I found "muriatic" acid (which is HCl) at a swimming pool supply store. It was ~33% concentration, which means it came out just right to mix 10oz of it with one quart of drug-store H2O2. It does a great job of etching.
It is way easier to use than FeCl, besides being easier to deal with disposal.
I tried the method you described (once), but it just isn't worth the effort. For about US$1 I can mix 42oz of solution, etch for a few hours, then dump the stuff. I'm still working on my first gallon of HCl (less than US$10).
It's just not worth the extra hassle of even storing the etchant to do the regenerated stuff.
-j