Well, looks like I found answers to my own questions, but in case anyone else cares, here's what I've come up with: http://www.fetrailers.com/about.html
Will galvanize an existing 21 foot inboard boat trailer for around $3500 (just ballpark). They'll sell you a new trailer for around $4600. They'll restore/recondition your rusting trailer, etc.
What was more interesting to me, though, was a company I found called El Dorado Sandblasting in San Diego: http://www.eldoradosd.com/
They have a Zinc, Epoxy, Polyurethane coating that they'll apply to an existing painted steel trailer. It comes out colored like a painted trailer, and is supposed to last longer than galvanizing. Is that possible? One guy I talked to (not at the company) said he's had this coating on his trailer since the 80's, using it in salt, no rust. They're saying $65/foot of length for the treatment, so that's way cheaper, and it usually takes 5 days to do, depending on how busy they are. You have to take the boat off the trailer first though... the guy says local boat yards will take your boat off, bunk it, and store it, for a fee...
Anyway, I thought other people might be interested in that.