I assume you want to write FRP in a Haskell-embedded DSL and generate FRP'd JavaScript. If you wish to use Flapjax as a supporting library I'd be glad to help.
I'm curious: how difficult is it nowadays for in-page JavaScript to control the evolution of its surrouding page, FRP-style? I used to do something like that, but with VRML+JavaScript instead of HTML +JavaScript (there's a screencast for those without VRML viewer;-): http://community.haskell.org/~claus/FunWorlds/VRML/ and I found that mapping FRP style behaviours and events wasn't the problem (though VRML has better support for this than HTML), the problem was gaining the same advantages one would have from embedding FRP in a functional language. Problems included scoping, linking dynamically created material with enclosing scope (JavaScript being in strings with scopes separate from the enclosing VRML, VRML to be created dynamically also via strings, also in their own scope)