LED_CTL is on GPIO 18, pin12 of the header
By the way, we are working on the Revision 3 of the KCeptor, inspired by the form factor and some improvements we added to the KCeptor++, but adapting it to the DIY procedure.
It will also require a new revision of the Hub (as HAT of course). We haven't started designing it. Would you like to do it? I could send you the schematics and you would have the PCB layout left.
Do you use KICAD?