Force Spotlight to index a normally un-indexed folder
Well, in order to answer the implicit question stated in your headline: You can force Spotlight to index unindexed folders by creating a link to them in an indexed folder. Say you want to index something under ~/Library/, you'll be able to do that by creating a link under your home directory.

Unfortunately, I can't answer your question proper: While this makes Spotlight comments etc. accessible, it still does not cause image dimensions to appear in Finder. But perhaps this is a starting point.