I don't own a bluray disc to check, but I think there might be a way to verify the release date or its code, or maybe use a bluray website database to know the release date of popular movies.
At the time of the bluray brick, the popular movie was Alice in wonder world. There's a big chance that this movie have the correct version number for the bricked PS3 reader. (it's just a supposition according to the date or the downgrade event).
Unfortunately, there's no way to know which played bluray disc had the greater version number if you don't have these discs to check the release data. Same if the console is a second hand, you won't know which disc the seller used on his console.
If I understood correctly, it's not the last played bluray disc which is needed, but the one with the greater version number the console ever played.