I have only one first-hand experience using lye but I did NOT heat the water (it was luke warm) and yes there was some foaming but I was only trying to strip anodizing from some AL parts and the process went a little further than that with some loss of detail to the metal (but that anodizing was REMOVED!). My guess is the foam could bubble up and out the top (that would be the BB shell) in which case the paint's a loss.
Another detail is that you don't need to
totally dissolve the post, just enough of the oxide that's chemically welded the AL to the steel (in this case both rust and aluminum-oxide AKA "anodizing"). But you can't direct the lye to go where you want it...
Did anybody use
Kroil as one of the chemical penetrants? When it comes to rust I find that Kroil will beat nearly all others (and I have tried ALL others!). Hard to find and costs more but when you're in a fight to win...
Plus the careful use of heat (remember your paint!) is also your ally.
Good Luck! keep punching!!