Have been doing this for various mods for the guild to install various mods including: GuildMap, CT_RaidAssist, LaVendetta BossMods.. as well as a large number of optional mods like TauntResist, SimpleTranqShot, Decursive, BuffAhoy, etc.
It's a pain to recompile every single time a new version of ANY of the mods comes out tho.. and try to keep a medium sized guild all on the latest versions.. *sigh* Let me just say, that I'm SO looking forward to seeing a verison of UniUploader that will do optional mods!!!
... In any case, you just need the NSIS installer package avialable here:
http://sourceforge.net/projects/nsis
Homepage:
http://nsis.sourceforge.net
In the script, it is hardcoded to pull the addon from your C:\Program Files\World Of Warcraft\Interface\AddOns\ dir, modify as necessary (It will autodetect the proper directory for the person installing the mods).
To use: Just rename the file to remove the .txt, right click and choose compile.
-Seph