Andy,
as you've found Newtonians are not designed for use with diagonals....
I think the barlow solution is well worth trialing at least you can then determine the best fl for imaging with your camera.
Check out the CCDCalc page - it allows you to see what FOV you'll get with various fl and cameras - there's a good selection of astronomical objects to allow easy comparison.
http://www.newastro.com/book_new/camera_app.html