I can edit the non-steam game and use the installed game path, but then it fails to launch because some of these games used to require that the game CD was present in the CD reader, obviously I don't carry it around and plug it in he Deck, this is why I've tried the same thing using Bottles but I'm not sure if I can easily mount a CD or ISO or folder this way, and only in the context of launching a game, not persistent. I have to look further into this and also check Lutris maybe, but if someone knows how to solve this, I'd be glad to hear about it!
For some games, it might be easier to patch it and remove the CD check...
For Re-Volt, a community engine and launcher exists, RVGL, which makes it way easier.