I've had success in the past with the o2 sensor adapters on another car..I had built a turbo system years ago (not on a Porsche unfortunately) that had deleted the primary cat, leaving only a secondary..that little o2 adapter worked great, no check engine light. Might work for your application, worth a shot they are cheap. Most of the cost will be having a muffler shop weld a bung in the pipe for you.
This is a pic of what I'm talking about..not this actual one but good reference
http://www.bigdaddiesgarage.com/images/o2-straight-adapter-install.gif