How can I leverage multiple iTunes libraries in a LAN to create an office jukebox?
You should be able to do this using shared libraries, which would probably be the easiest way and not cause any problems with permissions or anything like that, however it would probably end up with number of separate libraries rather than just having a one large library.

UPDATE:

Having had a bit of a look into the shared libraries feature it could be a decent option if combined with iTunes DJ - which can be turned on in preferences if currently off.

Essentially you would require everyone to turn sharing on for their iTunes library (or specific playlists) then have one of your computers acting as a master which has iTunes DJ setup. You can then add tracks from any of the libraries and have requests and votes for next tracks sent via the Remote app for iPhone or just shouted across the office.

May still not be quite what you're after, but essentially uses features already available in iTunes rather than a 3rd Party app to do it for you.

--

You could try having one folder for music and all linking to it, but you may hit permission issues or DRM or something silly like that.

http://support.apple.com/kb/ht1203

This tells you how to move the library to a shared folder for one computer to allow multiple users to connect to it, the actual library is still per user, so you may get duplicate files or have to update the library if new files are added. In theory this would allow multiple computers to connect to one collection of music, add to it and play the files others have added. Though my theory is untested and may be flawed (like all good theories).

On a different note:

I was able to have one library (so viewing the same apps, podcasts, music etc. without the need to update it each time you logged in or downloaded new stuff) for multiple users on one computer by copying the whole iTunes folder to the shared folder and then adding an alias to the shared iTunes folder in the Music folder for each users Home folder - which is a bit of a convoluted sentence - which may be of some help, but probably not as the library gets locked when it's accessed by a user, so i can't imagine it'd help for multiple computers. Just another way of doing things.

Hope that helps in some way, let me know id it's absolute guff though.