But MLLE is already here. You can go use it to make levels with right now. Whether it's worth the effort to re-implement basic JCS functionality is a moot point since either way that effort has already been spent. MLLE is functional, more compatible and flexible than JCS, and it's even open source. At this point in time figuring out how to hook into JCS would be re-inventing the wheel, since there's a perfectly functional editor that can do more than JCS already.

Anyway, ideas are a dime a dozen. Go start a JCS+ project if you prefer it to MLLE, if others agree with you they'll surely follow your example and contribute.