Upload was finally successful. The problem was that I was signing my packages (I had set up a key in GPG on my laptop, and it was being picked up automatically) and the repository servers had no way of validating it because the build servers don't have internet access to get to the key repositories. I found an obscure set of message board conversations noting this, so I thought I would mention it here in case anyone searches for it for hours like I did.

I kept receiving the message back from extras-cauldron-build that said:

If you did not provide a PGP key when you joined garage, you have no opportunity to provide one later. So you can't sign your packages.

Which means you have to build the packages with -us and -uc options, like this:

Then they upload fine.