Are there any adverse effects to using one for an additional 12ft of digital coaxial cable on top of 50ft of cable? I'd very much prefer a direct connection, however that isn't possible at the moment.

Wow, that's a long run - 62 feet! I've generally read people post that the insertion loss from adding a female to female barrel connector is so negligible that it doesn't matter. But I don't know if that is true over that length. I would ask an additional question, whether an optical toslink cable is better than digital coax at that length.

Oh, I just noticed from the title that it's for a subwoofer. OK, you meant an analog cable and not a digital cable. Completely different application. People use long runs for that all the time. No problem. And people often use multiple Y connectors and never complain about signal loss, so no problem there either with a barrel connector.