I spent a pleasurable hour last night designing a brake light flash circuit - used to spend my days with a pen in one hand and a soldering iron in the other, but haven't done any design for years - and came up with a nice one. Even got as far as a tiny PCB layout.
Of course, such things are available. I found this supplier:
Brake Light Flasher ...on my first search. Same deal, I'd bet it has the same circuitry as I sketched out. Splice it into your brake light line, connect the third wire to ground... presto, your brake lights flash.
He has a couple of models that flash differently. Partly because I live in California and partly because I like the pattern best, I designed mine to do the four-flashes-and-stop pattern; continual blinking is illegal most places (and annoying after a while), and I don't see any advantages to nine flashes before steady.
You could easily tuck this into the space next to the brakelight switch in the master cylinder well. Mount it with a bypass switch and you can easily disable it when you like. Thirty bucks, an hour's work - and presto, safer brakelights. Nice.
I'll probably still build my own, because I like to tweak things.