After much googling, I've found this:
http://users.telenet.be/miata/english/misc/lowfuel.htm
Which would seem to do the job - for an MX5, and the very last entry on this MOC thread has a circuit:
http://www.marlinownersclub.com/foru...read=666&cat=0
But that looks as if its more for calibrating mismatched senders to gauges rather than just the light. I assume I shouldn't have any issues in that area as my gauge and sender are matched (and you'd assume calibrated for one another) by ETB.