Curious, are you going to somehow control the propane as well?
It would seem, with a constant flow of propane, every spark after the first one is superfluous, no? I guess that you've figgerd all this stuff out, how to extinguish the flame between sparks and such, but, if the propane discharge is being controlled, whatever controls the propane would control the spark as well.
Don't think I'd worry too much about the unburnt propane floating down around the operator, the bicycle (hopefully) will be moving and the flume will just trail along like smoke from a train stack. Even being heavier than air it still isn't like a rock, takes a while to settle.
I think the auto flasher is the ticket, as you say and don't know but as with all car stuff, would imagine they'd be pretty big power hogs.
Bitchin' project, can't wait to see the photos