I managed to track a suitable cable down and I've now had chance to put it together and test it, and it works as expected. It was £19 from Amazon with next day delivery. Other USB programming cables for the "Kenwood TM-D710" radio may also be suitable. I already had a 5 Pin Domino in the parts box to replace the 8 pin mini DIN it comes with.
It makes for quite a neat cable:
As the cable colours don't follow any particular standard I had to "bleep" them out with a multimeter. To do this I cut the mini DIN off the cable leaving a couple of inches cable, and stripped each conductor. I then continuity tested each pin to match it to its colour.
Here's the pinout of the supplied 8 PIN Mini DIN. Note its as you look into the connector, which is different to some conventions but makes it easier as you'll be looking it from the front as you bleep it out.
These are the colours mine was using. Note it may very between cables so you should always check, but this might be a useful reference:
With the colour mapping in hand, you can now solder it up to the 5 pin domino like so:
Hopefully someone finds this useful.