The LED wouldn't have 12 volts across it in normal use for turning on the amp. When the empeg was powered on, it would still only have it's normal forward voltage across. The rest would be used to raise the sense-line on the amp, assuming the sense-line only draws a few mA. But it still wouldn't work, because if the empeg is turned off and the sense-line brought high by a switch, it would have 12 V across as reverse voltage and it looks like LEDs normally don't like more than 5V reverse voltage.

Stig