You've stumbled on to one of those "ye olde ancient silversmith's
tricks": Nevermind the firestain, just plate over it. I've done it
myself on a couple of chalices, and it works just fine with no
special prep work beyond making sure the finishing is perfect.
(Somehow, plate seems to accentuate scratches you never knew were
there.) For a truly wild effect, have them do a gold wash on the
inside of the bowl. A bowl that large will just turn into a lake of
golden reflection. (The wash is very thin, so don't do that except
for showpieces, or goblets.) I do it on my goblets when I can, the
effect is amazing.
Not to claim that I'm the slacker responsible for this little trick,
I picked it up at the Cass in London. It was pretty much SOP to
plate hollowware rather than worrying about the firestain.
As for borax and prips? Now that I think about it more, maybe it
would work....Couldn't hurt anyway, except that doing that would
distract you from finishing the joint and ending the problem
altogether. Generally, if you're burning out your flux, that's a good
time to stop, take a deep breath while you're waiting for the pickle,
and do it again carefully, rather than trying frantic 'off the cuff'
stunts while the metal's hot, and the torch is waving around
randomly. You might want to ponder using white paste flux when doing
large things that are apt to take a while. It hangs around a lot
longer than Prips does. Much preferred (by me anyway) for
silversmithing scale work.