Haven't done it personally, but I don't see why you wouldn't just drill and weld on a new o2 sensor bung where you need it, and cut and extend the wires from the rear o2 sensor to that point.
A local weld shop quoted me $45 to re-locate the O2 bung which also included a plug. I may do this this weekend, just need to see what it's goin to take to extend the O2 wires.
You may be able to find someone to do it cheeper, the shop I went to charges this amount as a minimum for any welding work.
In my case, it immaterial because I run the prodrive ECU and therefore, as you said, what would be the point. Except of course if you're running a unichip piggyback.
The original is behind the cat in the downpipe (cat #2), but in front of cat #3.
I'm interested as my autox class (STX) requires one cat to be retained. The third cat is the logical one to retain (or if you have the bucks-replace w/ a high flow one) as relacing the catted uppipe/downpipe will yield the most performance. The down side is you will through a cel (cat inefficeincy code) because there is no cat in front of the O2 sensor- unless one could relocate it behind # 3, thus the interest in the relocation kit.