Pete,
Check with the gunshop at Old Salem, Winston-Salem, NC. They recently built a pair of pistols using the TOW original brass pistol barrels (I believe you are using the same). If my memory serves, they threaded the low flash hole and silver soldered in a threaded plug. The results looked very nice, but when I saw them they were only partially finished.
If I were to do the job, I would tap the flash hole with a small diameter thread, like a #4 or a #6. Then make a brass threaded rod and silver solder it in place and trim off the excess.
Jim