Hal,
   I assumed there were paid reviewers, but my thoughts were that they are probably in short supply due to financial reasons and time constraints, as you stated, so if some how volunteer reviewers could somehow get a crack at these books it could help supplement the efforts. The differnce between the paid and the volunteer is, of course, that the paid reveiwer is held accountable and must meet deadlines to earn their keep, but the volunteers would be under less stringent expectations. Perhaps having to fill out scheduled surveys on their progress, wherein the surveys have specific questions on them. If they don't respond, they don't get their final release book and no mention in it. This is often done with beta software volunteers. With this in mind, the volunteer process is not relied on as the "main" mechanism for reviewing a book, but as a "supplemental" way of reviewing a book.
Perhaps I'm the only dope willing to volunteer? Just give me a free final copy of the book and a mention in it and I'll sign an NDA and give it a crack. ;) Um, of course, giving out too many free books to volunteers would have to be limited.