Without admin rights on our development machine, we can still install Fiddler into a private folder. However, that is not true for add-ons. The ones I have downloaded use an EXE installer and expect the target folder to be C:\Program Files\Fiddler2\... I would like to have the ability to install add-ons without admin rights.
FWIW, most addons can be simply copied into the proper folder, so you could install on a non-locked-down system and then copy to the desired (unprotected) folder on the locked-down system.
Generally, extension installers should be reading the HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Fiddler2\LMScriptPath and HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Fiddler2\PluginPath keys if they hope to install per-machine, or into the %USERPROFILE%\Documents\Fiddler2\Scripts and %USERPROFILE%\Documents\Fiddler2\Inspectors paths if they wish to install on a per-user basis.
We would eventually make this possible for the Telerik maintained add ons. But there is a vat number of third party add ons that will fail in this scenario.