For decades now, much commercial equipment, (like trucks, tractors, loaders etc. ) need extra revs to run hydraulic equipment, electrical equipment or PTO's. Most of them come with throttle kickers.
Usually its a simple pull-knob mounted on the dash. You pull the knob, which pulls on a cable attached to the throttle linkage somewhere and opens the throttle just a bit. . They are adjustable so pulling the knob gives you the needed extra revs and no more. Push the knob in and revs return to normal.
The better pull-knobs have a locking ring so they will hold the rpm regardless of how strong the throttle return springs are.
You should be able to get a kit to install one at a farm supply or farm tractor store, industrial equipment store, etc. You might need to be a little inventive in adapting it for your specific vehicle, but it should not be too difficult.