I ended up filling the body up with penetrating oil, and letting it sit for a few hours. I tapped it, vibrated it, and beat on it a little bit, every half our, or so.
In the end, I was able to ever-so-slowly work the rammer loose, by using a vise, dead-blow mallet, strap wrench, and some drifts/dowels (steel and oak).
It required a bit of work to get it cleaned up, re-polished, and moving freely.
I'm expecting a partial refund from the seller. It was claimed to be "complete" and in "working order". ...Neither of which is actually correct. It was missing the instructions, has the wrong powder measure, and was rusted solid.