Actually, the trick that worked for XIV also works for XI. Install base game and add-ons, copy old install over it, done.
But if you only want the reg key for your model viewer, I hope this helps you.
Code:
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\PlayOnlineUS\InstallFolder]
"1000"="C:\\Program Files (x86)\\PlayOnline\\SquareEnix\\PlayOnlineViewer"
"0002"="C:\\Program Files (x86)\\PlayOnline\\SquareEnix\\TetraMaster"
"0001"="C:\\Program Files (x86)\\PlayOnline\\SquareEnix\\FINAL FANTASY XI\\"
Paste this into notepad, adjust the path(s) if needed (keep the double backslashes!) and save it as something.reg. Double click, say yes, and your model viewer should find the game files again.