Is there a way I can get a unique identifier that is associated with a user's Lightroom instance (basically, an ID that is associated with their installation of Lightroom on their computer; for example, if they installed Lightroom on another computer the, ID would be different)?
Couldn't reall find anything too useful in LrSystemInfo.
I heard something about an undocumented LrUUID. Can someone provide me its methods / some references?
I think not. - begging the question: why would you want to.
LrUUID is just a random "number" generator (random mostly-numeric string to be more precise), it does not identify Lr instance.
Unfortunately, LrApplication.macAddressHash() is only implemented on Mac OS, not Windows.
You could use LrShell to run a platform-dependent shell command to get the MAC address of the first network adapter listed. On Windows, use the command "ipconfig /all" and then grovel through the output. I forget what the Unix equivalent is.