Not sure if it would work from what I can understand in how the site may function. Could be wrong... Mahz may be able to work it in.
Part of the reason comes down to the visibility of the posts on the site. On the public side every member can see public posts, GMs can see hidden posts in the threads, but no one can see deleted posts as they are completely removed from the "site indexing system" (best example is how bots are nuked and all post go bye-bye). Try it out on Google and search something like "roleplayerguild star wars".
That's the key point in a lot of this as the indexing system works in unusual ways and the PM system I don't believe is indexed in the same way. I don't know if a privacy wall would also have to equate into the search well as we wouldn't want an abuse of it if it somehow showed other people's messages.
