Found a great working example of keyword spotting with the in development version of PocketSphinx: http://syl22-00.github.io/pocketsphinx.js/live-demo-kws.html  Some of the words don't work well, but others like 'OK Google' seem to work very well.

This demo is from a Javascript port of PocketSphinx and unfortunately is limited to searching for only one keyword at a time.  However digging into PocketSphinx's source a bit, it seems the normal C library can be given a file of keywords.  More investigation is necessary (unfortunately the keyword spotting stuff is only in the subversion trunk and not yet documented), but it's good to see a working demo to know what's possible.