I tried it on both my jo (not brave enough to do my bokken). It is OK, but writing kanji with the tip of a soldering iron is a delicate art.
Practice as much as you can before trying it on your bokken. I haven't got that kind of time. Sadly. A brand would be more certain, but requires metal carving or casting, and in a mirror image at that.
I could think of some interesting alternatives, such as writing with thin, non-waterbased glue and dusting the character with gunpowder
Matches --matches -- I know have some matches here somewhere ...
I REALLY don't have that kind of time
Cordially,
Erick Mead