I've created two batch files the first one, copy.bat simply copies the second file, check.bat into every sub folder in the directory it runs in. (I copy both files then run it from d:/launchbox/games/sony - playstation 2) BEFORE you copy the create file into every folder, EDIT it and make sure the paths are the same that you use, if not, you will NEED to change them.
Ok, second file, check.bat, I use this as the rom file, import it into LB and choose "use folder name" when the option appears. I have all the PS2 games as folders so, for example, Gran Turismo 4 is : D:\LaunchBox\Games\Sony - Playstation 2\Gran Turismo 4 (USA) and it has a check.bat file inside it.
The batch file will check the directory it runs from to see if there is a .ISO or a .BIN file present, if not, it will download it from your remote storage location (I use my google drive to store PS2 roms) You would have to mount your google drive using rclone or something similar, so it has a drive letter, and again, check the settings of the batch file to make sure the location is correct. In my case, for GT4, the location is : Z:\redump\Sony - Playstation 2\Gran Turismo 4 (USA) and therein lies a .ISO file. It will give you an option to download to local storage and once complete, will display a message and exit. You can play the game by running it again from within launchbox.
NOTE - If you import your games this way, you then need to mass edit the emulator field and leave it BLANK otherwise it tries to use an emulator to run the batch file, which won't work.
Any issues whatsoever, please post and i'll check, but it worked perfectly for me so far, 1800+ ps2 games in my library but it only downloads the ones I want to play