This is a good idea actually. mIRC *should* list all the directories on your harddrive that contain the "mirc.ini" file, since this filename is hardcoded into mIRC, and always present, unless edited out.
mIRC doesn't seem to store the install directory path anywhere on the harddrive on in the registry. I have mIRC installed at c:\Internet\mirc\ for 4 years, and not a single update has found this path by default. It keeps defaulting to C:\Program Files\mIRC\ which doesn't exist.
Listing all the directories that contain mirc.ini would be useful, and possibly even give the option to have the installer update ALL locations at once... in the event you have multiple installations, or the Asylum or like-trojan installed.