Hi Nick,
that shoulnt be a problem.
the simplest way is to use a single pole change over relay.
The power, I presume, is fed to the pulsoid and the progressive will ground the other side of the pulsoid every time it outputs a PWM pulse ??.
So, if one side of the pulsoid is connected to 12v and the other to the progressive, you disconnect this from the progressive, feed the output of the pulsoid into the common of the single pole change over relay, then the normally closed output from the relay goes to the progressive, the normally open goes to ground.
when you apply 12v to the relay, the progressive is disconnected and the pulsoid is grounded - full on 100 % duty cycle.. when 12v is removed from this relay, the progressive is back in circuit applying pwm pulses again.
the relay drive 12v is fired from a micro on the throttle pedal.
there ya go..
drop me a mail if you want a circuit drawing.
Joe.