Pushing Large Files to 500+ Computers
Solution 1:
You really do want a Multicast File Transfer Program: UFTP, with decent documentation and proxt-style extensions for NAT/router traversal too.
Solution 2:
You might want to look into murder
Murder is a method of using Bittorrent to distribute files to a large amount of servers within a production environment. This allows for scaleable and fast deploys in environments of hundreds to tens of thousands of servers where centralized distribution systems wouldn't otherwise function
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Murder was/is developed by the folks at twitter, and they're using it daily to distribute files.
Solution 3:
A new solution may have appeared: BitTorrent Sync
Later edit: Actually these days I would probably recommend git-annex assistant or syncthing but there are many alternatives
Solution 4:
I might have an idea for you that would help. Forgive me as I don't fully understand why this needs to be so complex, but if your need is to keep it simple for the end user and yet make the data quick and easy to reach inside a LAN, perhaps you could go with a NAS device. I have a Synology DS1812+ it can RSYNC with another Synology drive or various RSYNC capable devices, it has a bit torrent capable application called "Download Station", I know you can download torrent files from the drive bay and I believe you can also create or post a torrent file with that application to allow others to download a file they need. It has apps for mobile devices both Apple and Android. It also can do FTP transfers. This drive bay could give you the ability to send a file to it quickly then disperse it amongst the LAN quickly and easily. I suggest placing the data inside the LAN just to make it faster access for the LAN users, but the beauty of these NAS devices is that you can put them anywhere online as long as they have a fast internet connection. Perhaps one of the higher end Synology NAS devices would be a good fit for your needs.
Synology has a virtual interface you can look it over to give you a better idea if this would be useful for you. I will paste the link below to the virtual interface
http://www.synology.com/products/dsm_livedemo.php?lang=us
This device also gives people the ability access their data via a web interface or mobile device application.
I hope this helps out and like I said, forgive me if I don't understand the question correctly.