For anons, it wouldn't be too hard to have a copy of the board local, running software like docfetcher to search locally for them.
There would need to be a halfway board where the records of the full board were updated regularly.
If docfetcher was used could the index record be downloaded from the server for any new user - this would take load off the server as it wouldn't be a true search run, would be based on a pre-compiled index file, na? I think docfetcher only incrementally updates indexes as well, and it would be easy to ensure docfetcher portable was the default tool to view, downloadable from the server…
https:// sourceforge.net/projects/docfetcher/files/