Our level creation workflow now looks like this:
1. Build level in 2D editor, export to data file.
2. Import data file and build 3D world in 3D modeling package. Export 3D world to 3D file.
3. Test in game.
Back to the 2D editor to make adjustments, until the basic level shape is okay. Then, we can add enemies and pickups to the level and test that some more. If that is fine we can finalize the level (make it look prettier!).
It might seem an awkward way to do things (and of course it would be really cool if step 2 was totally automated). But in fact, being able to make adjustments in a 2D editor turns out to be a real timesaver. Especially since it's very nasty to make size changes in 3D (keeping the mapping of the objects correct).
That's it for now!