[alistairsam]

ok, that's completely different from the eq3 hand controllers in terms of what can be done.
can you confirm if the speed controller dial is a simple pot?

What comes to mind is a simple microcontroller based approach that can either increase or decrease the resistance value of the pot based on the ST4 pulse. I've built a similar unit to interpret the ST4 pulses.

The ST4 commands are RA+, RA-, Dec+, Dec-. meaning speed up RA, slow down RA, same with Dec.
since what you have is a speed controller, you can do the same thing.

it goes like this. the ST4 port of a guidecamera or USB-ST4 has 4 pins that are normally held high (+5v). This is after an opto-coupler, so its safe. It is controlled via the Phd autoguiding software ( you set mount type to "on-camera").
So when the guide star drifts east, it pulls down the RA- pin asking the mount to slow down, or the RA+ pin if it drifts west asking it to speed up.
Phd determines the length of the pulse based on its calibration.

you then connect the 4 pins to an MCU (I use picaxe as its very simple to use). The MCU will then monitor the state of the 4 inputs. when a pin goes low, it either raises or lowers an output, your choice, and this can then trigger an opto-coupler that switches a fixed or variable resistor that is across each side of the potentiometer's centre tap or in series to the centre tap. or you could use a digital potentiometer.
you'll have to experiment for the actual resistance value or you could measure the effective resistance after turning your knob by your normal amount and then calculate.

The idea is rather than you rotating the dial, which just varies the resistance, you add or remove a resistor that's either in-line or in parallel to the pot.

Although it sounds complicated, you should be able to assemble this in on a general purpose board and the whole thing should cost you under $30.

if using picaxe, I'd recommend the 20M2 as it has sufficient input and output pins. opto-couplers are only a few dollars each.
i'll see if i can draw up a circuit. I have already written the code for the ST4 interface for the picaxe, so let me know if you need that.
Cheers