Just in case anyone else thinks that the above sounds like a good idea.... Here's why it's not.
I used a couple of very thin knife blades top and bottom on one side to ease the 'ramps' path, and a thicker blade at the side to pull the cluster out past the springy bit on the side.
Now, I'd taken the back off and removed the circuit board, and was just about to start hacking holes in the case to glue two of these into place behind the 'dead' buttons.
When it occurred to me that I'd need to make corresponding holes in the circuit board. Maybe not such a brilliant plan.
I might revisit if I cba to do it with pcb mount switches and external relays. In all likelihood I'll probably just bash then through the lower dash somewhere - trying to be a grown up and doing it tidily was too much of an ask.
The plan had been to use the switches to fool the audio head unit into not dimming when it's broad daylight but I need lights on, and while I was at it, it would have been easier to do a second one instead of routing a wire down the the handbrake switch to let me use the deeper menus in it.