SBT AutoPlugins FTW! Put this in your global sbt plugins directory to get an "openHere" command to launch the MacOS finder in the project directory.

OpenFolderHere.scala

import sbt._
import sbt.Keys.commands
object OpenFolderHere extends AutoPlugin {
    override def trigger = allRequirements
    override lazy val projectSettings = Seq(
        commands += Command.command("openHere") { (state: State) =>
            "open .".!
            state
        }
    )
}