This is a great question and it's one that we're hearing more regularly. The short answer is no, this isn't currently possible. TubePress's functionality is largely limited to what is offered by the YouTube API, and they simply don't offer a way to search within a playlist. The best they offer is to restrict search results to a given YouTube user/channel, which isn't quite what you're asking. So this restriction isn't specific to TubePress; any software using the YouTube API has the same limitation. Does that answer your question?
p.s. I live in San Diego and am a big craft beer enthusiast, so naturally I'm curious to know if you're building a website about San Diego beer? If you're local, we should grab a pint to discuss!