Doing the form submit with GET didn't work in IE (6 with SP2), so I switched it over to POST. Now everything is wonderful, except:
Sometimes it takes an hour to input the data for this monster form. Problem is that the site parameters have logins timing out at 15 minutes (this makes sense, since many students accessing the site will be doing so from shared computers). So there's a decent chance of the person doing the input on the monster form getting logged out before he's completed it. The problem is that the redirect to login results in the monster form post getting turned into a GET, with return_url=something-about-a-million-lines long. That GET is too long, so when the user hits submit from the login page, the browser throws an error. (Happens in IE and Mozilla both, although the exact symptoms are slightly different.) Depending on browser settings, this can mean an hour's work down the tubes.