Well, I kinda succeeded in altering the buttons on the Tasmota UI, not quite what I was after, but it is a start!

I'm not sure where the fine dotted black lines have come from on the 'Toggle' button, but at least the UI can be altered, even if only a little.

I used Atom to create the custom firmware, as per a hint from coderus. My only gripe with Atom is it causes 100% CPU usage. My system is not short on resources, yet it still takes around 30 minutes to compile the 520kb firmware.bin file.