adaliabooks: If you're just looking for the IDs then I think your brute force scanning is the only way to find them short of asking people who own the bundles (which I believe is how mrkgnao did it for MaGog)
WinterSnowfall: *Sigh* Yeah, guess I'll have to prepare myself for a 4th voyage on the product API oceans. Who knows what other things have shown up behind our backs anyway - might as well have another look. I bet Johny will be happy :).
Someone drew my attention to this.
Here is how I did it:
1) My primary source was this: [url=http://www.gogwiki.com/wiki/User:Barry_Woodward]http://www.gogwiki.com/wiki/User:Barry_Woodward[/url]
This is the GOG wiki page for Barry_Woodward's library (i.e. unbundled). If you look at the source of the page, each box has an ID (courtesy of Barefoot_Monkey), the unbundled library ID, which is what you're looking for. Barry_Woodward buys practically all GOG games upon release, so his library is as close as it comes to the entire GOG library. The only drawback is that he updates his wiki page only about once a month or so. If you're in a hurry, you can PM him to ask him to update it (I never did, as I'm the shy type).
2) An alternative source was this: [url=http://www.gogwiki.com/wiki/Special:GogExtensionData]http://www.gogwiki.com/wiki/Special:GogExtensionData[/url]
This is a page that Barefoot_Monkey set up primarily for MaGog, I believe, in which he collects all the IDs ever seen by Barefoot Essentials (when people use it to upload their library or wishlist to the wiki). It's a mix of catalogue and library ids in no particular order, so one just searches through it to see what's available. However, since it depends on people uploading to the wiki, which --- excepting Barry_Woodward --- less and less people do (and even if they do, they don't necessarily have the newest games), it's not much more useful that the first option.
Hope this helps.