OK I tried a search and found nothing.

I am converting to obdI (I have a 98 civic). I plan on running a test pipe. I wont throw a cel will I since the ObdI only checks on of the 2 o2 sensors?

 

i got obd2 and no CEL without a catalytic and 2 o2 sensors.
aslong as your car isn't running pig rich you'll be ok.

 

I'm OBD1 and only have 1 O2 sensor.

 

OK cool, that answers my question

 

I am OBD2 with a "resonated test pipe" and I throw CEL #67.

Catalyst Operating Below Effeciency.....

 

There's a simple fix to this, make a buffer amplifier that takes the signal from the one O2 sensor and attenuates it, then feeds it into the intput for the second O2 sensor. The ECU will see some activity in the tailpipe but never read rich on the second sensor input.
Maybe you could just use a resistor and a germanium diode to ramp the O2 signal into the second O2 input...