It's a very interesing idea, but I think if you want to *fully* support JJ2 levels, you'll need to support a lot of other JJ2 features as well. The two games have extremely different styles of movement, and JJ2 levels are often designed around special moves (which the jj1 sprites won't allow for) or general quirks in the game. Basically, some (maybe even most) levels will probably work fine if converted to JJ1, but there will be a number of cases where they are incompatible.

Still, what you have going right now is really good, and a lot easier than recoding JJ2 would be. Plus, it can work as a good starting point for adding more and more compatibility with JJ2 levels, perhaps to the point where there'd be no need for anything like an "OpenJazz2".