I am FAR from an expert at this, more of a dabbler really, but torch
enameling can be done and in my experience is preferable to kiln
enameling. The process is much the same as kiln enameling but when
it comes to heating, you have to heat it VERY slowly initially and
the enamel will eventually go black, then heat quickly until the
enamel goes glossy (or just before that if you want the orange peel
level for further enameling.
I do it with a standard propane torch (not even oxy).
I also usually try to avoid heating the enamel directly, so usually
go for a very thick backing.
The one problem I have found with it is that the colours are
slightly different than kiln firing and I find it VERY hard to get a
good clear enamel. It usually goes a little yellow.
I hope this helps.