We have water!

While at Home Depot, I actually asked someone in the plumbing department about my cap situation. He suggested that since it's a short-term item, and I don't care about appearances, I should use a compression fitting and a plug on the end of it.

So I did, and after a false start (not enough Teflon® tape on the plug threads), we've got a solid seal and can use our plumbing.

There's still a Pavlovian response going on though. We're expecting to find it leaking RSN but I'm guessing we're solid at this point. It's a mechanical seal now, not an epoxy/chemical/voodoo solution.