Simplemachines Forum
http://www.simplemachines.org/has it.
basically the mod files need to have a certain structure, and then they are parsed and joined with the exisiting board files.
The process is automatic.
If you download the source,
http://www.simplemachines.org/download.php?d=smf_1-0-rc2_install;t=zipin the folder Sources, the file Subs-Package.php contains all the file parsing and replacing stuff.
I think it would be relatively easy to modify this to be useable for UBBThreads.
Opinions?