Rewiring the INA149 diff-amp for 20:1 attenuation instead of unity gain

I'm interested in monitoring the voltage from a +/-135V supply using the INA149 in a novel way.   If you wire the INA149 pin 1 (REFB) to pin 6 (VOUT), and REFA to "GND" (center of the +/-135V supply), then the INA149 provides a 20:1 difference attenuation instead of the usual unity difference gain.  I would power the INA149 from +/-18V since the output would be 270/20 = 13.5 volts.  The datasheet doesn't cover this application.   The INA149 would dissipate the same amount of power as if the inputs had a 135V common mode voltage, so that isn't a problem.  I think the question becomes what is the breakdown voltage between pins 2 and 3.   Anyone from TI care to comment.  Thanks, Steve