My printer is currently in pieces so I can't test, but I assume the way to do it with the meta commands would be a macro with basically this:
M409 K"sensors.analog[<sensor number>].lastReading"
while result >= <shut-off temp>
M409 K"sensors.analog[<sensor number>].lastReading"
M81 S1
Is this correct? It would busywait (which probably isn't important). Is there a way to use them to actually influence M81 S1? (so rather than having a macro to do a delayed shutdown just be able to put M81 S1 in the console)