I started hacking together a plugin based on sbt-purescript. I could make a repo on github if you're interested in working together. My biggest stumbling blocks are that I don't know much about working with Dart and I've never made an sbt plugin before. :)

That's great! I'm in the same boat as you are, never made a sbt plugin before and especially not one of those sbt-web ones.

I'm afraid though I don't have much time to spend on this at the moment so I'll have to defer learning this until later.