From the driver's perspective, the 85+ electronic speedos all look the same as the 81-85 mechanical speedos. Cruise control was offered as an option in OZ as far back as 1982 - I had a dealer sales brochure for it once but lost it sometime last century - and this was back when OZ cars were either K-Jet or carb, all with mechanical speedos.
I stumbled onto this when I was looking at how they did cruise control on pre-81 cars (with the early cluster) because I wanted to know how they did the VSS, which I needed for the LH2.4 swap into the GT.
I guess the easiest modern way to do cruise control with a pre-81 speedo is by using a Brantz BR4 sensor connected between the gearbox and cable to the speedo to supply a vehicle speed signal, together with a later style electronic Cruise Control Unit, and adding some pulse conditioning hardware between it and the BR4.
The BR4 supplies 4 pulses per revolution at the speedo end. It's not rocket science to knock up a pulse conditioner / pulse multiplier circuit using some 74xx series TTL chips to give an output signal that matches the requirements of whichever CCU one decides to use.
The tricky parts to get will be some mechanical items, like the throttle spool.
