Just a quick update.
ALWAYS USE PROTECTION... I should have known better really but I got lazy and there were actual flames, but no injuries!!
Connected the 24VAC GND to the DC GND and it really does not like it when using a bridge rectifier.
After a small moment of panic and further investigation, it appears the only damage was the bridge rectifier, primary side rectifier PCB tracks disintegrated and the two tiny jumper wires I was using connected directly to the 24VAC transformer were the items that caught fire.
Most importantly the Espruino is still okay, its still relatively isolated being powered via USB at the moment.
Next week I will replace the bridge rectifier with half wave rectification, use some protection on the secondary side of the transformer, make sure the wires are suitably sized and try again. In theory this should do the trick.
I have finished adding on all the input connectors and hooked up a thermistor bead on each input. They at least seem to be working just fine now.
Need to track down a 4-20mA sensor to test. I'm sure I had one kicking around!