That's true what you say about the coatings on newer bulbs. The coating might accept the dye w/o any gelatin whatsoever.
I'm not sure on the use of slide film for the flash. I'm personally more interested in the violet stain for panchromatic materials just because I don't mind if the flash is totally invisible, just not "dazzling"! (haha, I love their use of that word)
Coupled with a flashbulb and you've still probably got a sizable guide number. Alternatively, taping a Wratten 34, 34A or 35 gel over a strobe would probably have the same effect, based off of the transmission curves.
I might try to get some methyl violet on eBay, but the supplier says that they reserve the right to ask for your company affiliation, so the same roadblocks might apply even to eBay. But who knows...