As for getting people to install the plugin, this wouldn't be a problem for me as I would be intending to provide it with with a version of Mozilla Prism that I would be distributing as part my project.
Well sure, and given your line of work, I kind of anticipated that this might be the case for you, but most people wouldn't be in that fortunate situation.
I tend to agree with Winni that a BlitzMax to Flash compiler does sound like the most likely bet, if any bet could be considered likely. I can see there being a moderate market for that, given Flash's huge market penetration, and BlitzMax does have a nice syntax.
The question then becomes whether BRL would need to sanction it or not. They wouldn't have to write it, but if it was going to borrow heavily from their syntax, I expect most developers would at least want their permission.
For what it's worth, if there was a BlitzMax -> Flash compiler, I'd buy it. Be nice to be able to code an interactive website in BlitzMax if I could anticipate most people being able to use it.