The hole in the lid is a standard 1.5" triclamp. I got one of these for closed transfers:
1.5" TC TO BALL LOCK GAS POST
You can clamp it on at the beginning of fermentation and purge sanitizer out of your keg with a ball lock jumper. Then when you're ready to keg, leave the jumper connected, run a hose from the lower valve to the liquid post of your keg, and open the valve. Bam. A closed transfer with free CO2.
Interesting. So let me get this straight. You dump the CO2 from fermentation into a keg filled with sanitizer. I would presume that the keg has a liquid line that is dumping the pushed sanitizer into a bucket. At the end of fermentation, the keg is empty of sanitizer and filled with CO2?
How do you pressurize the GFC at transfer? And I presume you'll need to dump the rest of the sanitizer from the keg by pressurizing from your CO2 bottle? Your solution sounds cheap and, honestly, brillilantly simple.