The net result of lamps will be to reduce the voltage to the motor - because considerable voltage drop will be taken across the lamps. A single 120 volt, 100 watt bulb will present considerable resistance, as compared to the motor so most of the voltage drop will happen across the bulb -leaving little for the motor.
If you think of the 2.5 hp motor as a 2,000 watt bulb - then a 2,000 watt bulb in series with the motor would put 1/2 the voltage at the motor. All of this assumes the motor is fully loaded and won't work out exactly this way - but it illustrates the order of magnitude.
My knifemaking pal took the motor and controller off the treadmill and kept the wiring as it was.
Unless I am mistaken you are essentially starting with a 20 amp or so 130 vdc supply. Then you need to vary either the voltage - or shut the 130 vdc on/off at a fast rate so that you can control the average amount of power going to the motor. Pulse width modulation is one simple way of doing that.
A crude way to accomplish this might be to purchase a 20 amp variac and rectify/crudely filter the voltage. There are certainly better ways to do this but at this power and voltage the project is far from simple.