I love it, that's a cool idea, especially for a big club! I look forward to build details, I might want one of these as well! 
I think the command base is the right way to go, it's already working hardware
You can make the test even easier. Just step the signal up in small increments and generate a horn/whistle command, if the horn blows, the engine is receiving. Probably a whole lot easier to detect than the flickering light, that might be a bit tricky to interpret. You will want to wait a couple seconds between each test amplitude anyway as the TMCC logic takes some time to react. There is a dead space of around a second between commands where additional commands are ignored. This is because TMCC sends multiple commands to maintain reliability, so there has to be a pause between different commands. It's also why when you blow the horn, it doesn't stop right when you let go of the button.
