I've implemented this using
<category> tags in each feed item (job). So if a job had the tags javaspringjpahtml and css, the feed item for that job would look something like this...
<title>Web Developer at BiddingForGood, Inc. (Cambridge, MA)</title>
<description><p>Do you want to make a difference in the world? Do you want to be part of one of the fastest growing tech companies in the Boston area? Do you want to help create a service that gives charities and schools $6 back for every $1 they spend and has raised over $200 million for thousands of worthy causes?</p><br /><p>The BiddingForGood development team is looking for a new Web Developer to help take us to the next level. We provide online auction and other fundraising tools to nonprofits. We're a small, friendly team that focuses on delivering great software to our clients without a lot of process overhead.</p><br /><p>Position includes competitive sick/vacation/holiday package, equity, health, dental, long-term disability, life insurance and 401k plan. &nbsp;Sponsorship and/or relocation assistance is not available for this position.&nbsp;</p><br /><p>All resumes should be sent to firstname.lastname@example.org. Please include salary requirements when applying for this position. Email response only. We are an equal opportunity employer.</p></description>
<pubDate>Tue, 18 Jun 2013 16:39:16 Z</pubDate>