I think it would be a good idea to ask the audience of PG to help looking for missing pages. This would certainly increase the chance of recovering them, and us to post complete books.
However, I am strongly opposed to posting incomplete books. If the public can not be sure whether the books they download are complete or not, they will move on to a place where quality can be guaranteed. I think in these kind of issues quality should prevail above quantity.
PG already issues books with missing pages, e.g. #11866. However it is stated at the beginning that certain specified pages are missing, so the reader knows what to expect. If the currently best available copy of a text, which may be several hundred years old, is missing a few pages, well that is unfortunate; but surely it is better to give people the chance to read the 99% that is available. Our great museums do not say, this pot has a few chips in it so we will not exhibit it. However, possibly there could be a list of PG works that require pages, so that there would be a higher chance of someone eventually contributing the missing pages. Philip.