Just tried it with a Harmony Cart on my 2600jr. The first screen seemed to jump up and down a tiny bit, but that settled down after a while, so maybe it was just my system. The only suggestions I would make are: 1) 8 directional movement, like in Halo 2600, and 2) maybe do something with the top half of the screen when it's not in use, if only to avoid confusion (although I'd be hard pressed to suggest what).
Other than that, it's a great little game! Had me thinking of Cloudy Mountain on the Intellivision. I'd be interested to see what the box and manual artwork would look like for this.