Reading through this thread, the fuel idea def wouldnt work, it primes which would stop from starting, and as soon as you go to fire it up, the pump will run killing it again.
Best signals i can think of would be the alternator output (on the dash this controls the battery light), or the low voltage side of ignition coil - possibly, not sure how they work if its a constant live and a signal feed of when to spark.
Oil pressure light also could do the job
IF using the warning lights route, youd just need to be careful on the relay, would need to pull a very low current for the relay.
Also....are dash light leds or bulbs? 12v?