Evan,
There are generic O2 sensor available with grounds attached, check archives.
Here is a page on sensors: http://www.wps.com/LPG/o2sensor.html
I have some info on my page (below) and I have a few O2 sensor monitors too.

Now, from the fear monger's department:
Regardless, I would ensure the engine to chassis ground is good. If you add
a grounded sensor and the engine ground is bad; the alternator current might
induce a voltage across this new path. Which, conceivably, add or subtract
from the sensor's output voltage
.
Ken Lewis
http://neksiwel.20m.com/