squid830: Does the GOG installer actually check the registry values, or does it simply use them to identify the location to install into? If the latter then you should be able to manually set the directory when the installer suggests/asks for a location.
Gydion: I believe it checks. In fact I'm not entirely sure if GOG will let you purchase it without owning the base game. I seem to recall that glitching with MindGames DLC (they actually owned the base game).
Seems a bit strange for GOG to do that.
I can fully understand that GOG take advantage of their own versions to locate the game, although it shouldn't be strictly necessary it does make writing installers easier across a multitude of games/platforms.
However, for GOG to <i>require</i> this seems a bit much. At the very least it could mean they miss out on some sales (from people who've bought the original somewhere else but now want to buy the expansion from GOG). In other words, it's a case of some developers somewhere coming up with something that seems like a good idea (since it may reduce their development time with respect to installer writing), but may inadvertently be having a negative impact.
It's also leaning dangerously towards the kind of publisher/distributor lock-in crap that makes people (such as me) hate Steam so much. Since if you're using the installer that comes with the game (I suspect most games, if not all, would have their own installers), or if you're copying the files manually, it would work.