Solution: add /Users/[user]/Library/Caches/Metadata/Precipitate to your custom sources in the Quicksilver catalog (or add the next folder down, if you like.) Under source options set the depth to 3 (or 2 if you went deeper.) Then, for good measure, rescan the source. Voila! Google Docs and bookmarks show up in your Quicksilver queries, and they open in your default browser as expected!