Unreal has this file type that has the extension .ut(insert version number here)mod, and what is does is install a mod onto a client's UT installation with no problem. I thought this mod've used one of those.
In reply to Jeep - it's just like a .exe, but it tends to give less errors and is more end-user friendly. It's also a recognised format amongst UT gamers, so people will know what to do with it (Open it, and follow the on-screen prompts)
In reply to R315r4z0r -
They would have to download it in the same way you downloaded the UT3 demo.