"Could I somehow wire into the thermocouple?"
Not unless there's something that will kick up the DC milli-volts generated to whatever a light needs to glow. And a practical way to do it and still keep the same gas valve.
Cob up a Mercury flame sensor or a Baso Pilot operated switch. You'll need to keep the existing pilot burner and thermocouple. Rube Goldberg the Merc flame sensor or the extra thermocouple that you'll attach to the Baso switch. Somehow, to the pilot burner. So, that the pilot flame will impinge on it. Or buy a pilot burner with more flame outlets, if need be. Pilot goes out, switch opens or closes, depending on what you buy. Wire the terminals to a clangston, siren or flashing disco light, if so desired. By the time you buy the add on's. You could have ordered the valve with electronic ignition...How about this. You're using some kind of temp controller to fire the boiler. Right? If the controller has the capability and a low event set of contacts. Hook your light or other gadgets needed to get power to the light, to it. If the boiler temp drops below set point, due to wind blowing out the pilot, or running out of barbecue tank fuel. The switch will open or close, depending on the arrangement. I use the same valve that's in your rig. I haven't had a pilot failure. However, I use NG and brew indoors. I stay near the boilers when they're fired, anyway. But, if you're going to play Bingo or clean the house, while brewing. You may consider a back up siren and phone dialer. Just in case.
