I'm modifying my old CG5 (stepper motor version with dual axis drives) to add an ST4 (autoguider interface) via connection of a RJ12 cable to the momentary pushbutton PCB.
Electronically my CG5 controller is identical to your EQ5 dual axis drive handpaddle.
It's easy , all I've needed to do is drill a hole in the side of the box, and I'll the need to solder all but one of the wires in the RJ12 cable to this , I have to replace the RA-plus ad RA-minus pushbuttons first though as they have been broken accidentially and when I disassembled the controller .... ooops!
Word of caution .... be gentle levering off the plastic tops of the pushbuttons, it is very easy to break the connector on the top of the momentary switches , and they are hard to find replacements for (I had to by my replacements from the USA ) , and be gentle when disassembling and working on the disassembled controller as the solders to the ribbon cable are not great , nor is the strength of the wires in the ribbon cable between the main PCB and the pushbutton PCB (I managed to break nearly half connections to the pushbutton PCB and desoldering holes with remnants of wire in them was not easy to do (don't have the right tools or skill needed) and it was super fiddlely and very fustrating resoldering the broken wires back on.
I'm following the method to do this layed out in detail by ShoeString Astronomy
http://www.store.shoestringastronomy.com/eq_mod.pdf .
I've recently acquired an old ST4 , which I'll use to guide the CG5 which willl become a dedicated autoguided camera platform for widefield imaging .... don't need a computer interface with ST4.
I've also acquired an Orion Starshoot Autoguider which I can use too , but requires a computer interface (the guider has the interface), I'll mainly deploy the SSAG on my New Atlux for prime focus guided imaging via my OAG. It uses PHD Guiding.
