finkleroy: That's a good question. I'd also like to know.
Ancient-Red-Dragon: Would you perhaps be willing to test that out and then let us know what's up with that? :)
I tried the music restoration mod and the 3D vision fix mod for Yakuza 0. Both work.
I tried the music restoration mod and the Hiroki Narimiya Restoration Patch (Restores the performance of an actor who was replaced because he made his own candy. Apparently that's a cardinal sin in Japan). Both worked.
The music restoration patch for Yakuza 6 works, but the green filter removal mods do not because the Ryu Mod Manager gave me the error "Warning: Game version is unsupported. Please use the latest Steam version of the game. RyuModManager will still generate the load order, but the game might CRASH or not function properly". When I attempt to launch the game with the mods installed, the game refuses to load or give me an error. So as far as I can tell, mods do work with the games on the old engine, and as for the Dragon engine games (Yakuza Kiwami 2 and Yakuza 6), mods that don't require the Ryu Mod Manager work, but mods that require the Ryu Mod Manager don't work because the Ryu Mod Manager needs to be updated to support the GOG versions of the games.
That said, someone said that a free cam mod didn't work, though I don't remember what game it was for. If it was for a Dragon engine game, that would make sense. If it's for one of the other games, that would indicate that some mods may work with them and some may not.