Janet Notman in UK does this very successfully on copper beads.
She uses Mapp gas (which is hotter than propane or butane) and
clamps the torch to the bench.
Then with two mandrels - WITH a stop on them to stop the bead
burning the hands! - rotate the bead in the hot flame (think of a
bicycle movement). When the copper bead is red hot, rotate it in
powdered enamel which you have placed handily nearby. Then fire in
the torch flame. Repeat.
Apparently the movement is similar if you make lamp-work glass
Alternatively, use Klyr Fire glue and wetpack the enamel on to a
suitable metal bead (usually copper)
Torch firing will distort the colour of some enamels.
Here's a link
or see the Guild of Enamellers website