It's just light surface rust, really.
Anyways, as it turns out, it still works. It was laid up with the ram entirely retracted, and pumped up readily. Could not move the release valve, but some drenching with Break Free and some gentle coaxing with a hammer eventually got it to where I could rotate it with the handle and, eventually, got it free enough to let the saddle come down.
After soaking a few more places, to reduce the creaks and groans, I managed lift up the front of a V6 Camry and it did not leak down. I only watched if for a couple minutes, but, not bad, at least for now.
Not exactly a "rapid pump", but least I can save the cost of a new jack, for the moment anyway.