I think the best way to do it is to install a new bung and wire up a dedicated O2 sensor. The bungs usually only cost $15 and a weld shop should be able to install one for 20 bucks or so. If you know electronics you can add a buffer stage between the O2 sensor and the A/F gauge do you dont change the impedance of the line.
A Tune does not make it not smog legal as long as you dont disable systems. And the only system I believe that would violate this would be if you disabled the EGR system.
I would get a triple and measure you oil pressure also...
2001 ZX3 2.0L Zetec-
Hatch Nation #42
Going through Rebuild See "The TICK Build Thread" for details (http://www.focusfanatics.com/forum/showthread.php?t=289168&referrerid=62897)