I came up with a little 16k bankswitch template (posted over in 2600 Programming forum) that I have now copied all my original code into. It's working great - I put a dummy 'Atari Rainbow' title screen that then skips to the main game when you hit the Reset button, and my sprite drawing routine has been shuffled off into banks 2 and 3.

No screenshots though as it looks exactly like it used to - but that's a good thing!