Jimpie -
So just out of interest for me. Is there something akin to my idea or should I send off the patent now?
Your idea would work, but it is an unnecesary and expensive complication which requires a ground installation.
There are already simple autonomous features built into any aircraft (even knackered old 1950s spamcans, let alone a state-of the-art FJ) to remind forgetful pilots to lower the gear.
For reasons best studied in a human factors course, even the best trained pilots still occasionally manage to forget the gear, and then manage to remain oblivious to the warning signs. Adding another expensive system won't necessarily remedy the occasional wheels-up landing.