Well, now that ship has come and gone, I can more fully take stock of how it turned out.
We wound up riveting our entire frame using 1/4" rivets; the only places we took them out were places where we wanted to mount other things with bolts. (I'm starting to believe Paul and his speed claims after making those changes.)
The result held itself together well through weeks of inexperienced students working around them and some bumperless bumping around the hallways of Columbia High. We didn't have a chance to see how the frame holds up to competition-grade whacks, but I remain pretty optimistic. I'll wait on the Chesapeake postmortem before declaring it a success, though.
The best picture I have of our handiwork is seen at
http://www.flickr.com/photos/billfred/2283099106/ that photo is from about a week before ship day.