Guess no one else is wondering about this…

Anyway, I’ve been doing some testing. Obvious note: I’m testing using test catalgue/library files. The below is based on my experience only. Be careful trying any of this as it may stuff up your library/catalogue!

To get Lightroom to include the Photos App masters folder, I opened the Photos App package contents, then dragged and dropped the masters folder into the Lightroom Library view. Lightroom will then add the folder in the left pane. Now all you need to do is right-click and synchronise the folder to get all the photos to appear in Lightroom.

So far, everything seems to work fine, except for:

1. Deleting files through Lightroom doesn’t work. Whilst you can actually get Lightroom to remove the file from your disk, Photos App doesn’t seem to register this at all. It still seems to think the file is there, even after restarting the app. I had to “delete” the photo through Photos App (and permanently delete from “Recently Deleted”) for it to actually disappear from Finder.
2. Whilst Photos App file structure is by YYYY/MM/DD, this is based on the import session, rather than the photo capture date. This is unlike Lightroom, where on import I get Lightroom to put the imported photos into YYYY/MM/DD folders based on capture date. The issue this raises (for me anyway) is that I like to browse the folders in Lightroom to find photos. However, since I am synchronising to Photos App master folder, I can only see photos based on their import date into Photos App. Bit annoying.

Apart from the above two issues, it seems to work ok. I can manage my photos through Lightroom, but still have them sync to iCloud.