...It may be possible to extract the GOG patch contents and manually update the non-Steam DVD version, as well...
From what I've seen of GOG's updaters, this wouldn't be impossible but very challenging. Aside from GOG-specific registry entries, their updaters typically (but not always) check each file present, and may abort if even something trivial like a readme or intro video file is missing. Plus you'd have to have the GOG version of the game to download any updates in the first place.
I'd agree with Dancingcrab that an update for the DRM-free disc needn't be seen as urgent - but making one shouldn't be much harder than running WinDiff (or similar tool) between latest build and disc copy. Larian presumably have an automated system for this (if not, they really need to put one together!) and they will need to do this at some point for the Larian Vault version anyway.
