The VHF a/g frequencies are allocated (by whoever allocates the electromagnetic spectrum, ie not the CAA) for voice comms only. PCL is signalling which (surprise!) is a different classification to voice, ergo; illegal.
What if you had a lighting system that simply came on if any voice transmittion on the frequnecy lasting longer than 3 seconds (to avoid random interference).
So you announce on the airfield frequency that you're inbound from the west, and when you get there the lights have already been turned on! No illegal signaling....just normal radio calls.