On our fire apparatus we use what is called a vernier throttle. It can be mounted anywhere, ours are on the pump panel and used to control engine rpm's, hence the water pressure at the pump outlet. They can be found on wood chippers and other applications.. Yours could be mounted on the frame along with your other controls.
The cable is moved by rotating a knob in a threaded housing so the cable is capable of pulling a lot of pressure. We usually weld an eye to a piece of the existing throttle linkage and run the end of the vernier cable through the eye with a stop.
