Secondary Mirror Too Small: Regain Lost Aperture With An Barlow?

[Update 2013-10-13] On second (third? fourth?) thought, I don't think this works like that. The Barlow lens would need to be before the secondary to recover lost aperture.

By now, I have two Newton telescopes with an "undersized" secondary mirror (won't illuminate full primary mirror), and I just realized:

If you use an Barlow with these scopes, then more of the primary becomes visible.

The closer the Barlow's lens is to the secondary, the more pronounced the effect is – so a "Shorty" Barlow is not as good as one with a long barrel. (Though the optimum for spherical mirrors would be a good "Bird-Jones" corrector.)

And one needs to make sure that the Barlow itself does not introduce vignetting itself…