I have a suggestion. Run every other paragraph through a Borkifier engine as a preview copy. That will let them see the approximate formatting and verify that you did the work of formatting their book, but the result will be completely unshippable to customers and won't be machine-translateable back to the original (assuming you make your Borkifier slightly lossy). Release an un-Borkified copy upon payment.
For example, here's a quick hack:
http://homeserver.gatwood.net/borkify.pl.txt