You might could use a
jet pump, along with a turbocharger configured as a
jet engine. Hook up the turbo exhaust to the nozzle of the jet pump, and connect the car exhaust to the jet pump inlet.
Jet pump
Turbocharger jet engine
I'll leave it as an exercise to the reader as to how practical this device would be.
Update: Make sure also that you provide some sort of correction factor to your O2 sensor outputs. They were designed to work at around atmospheric pressure (or slightly above on the exhaust stream side). Subjecting the exhaust stream to vacuum will cause your O2 sensors to start reading richer than actual, due to the way O2 sensors work.