I made my own rack frames from 3/4" EMT conduit. It's a nice snug fit in the mounting holes, so I don't have any wobbling and rattling going on, and I didn't have to do anything to secure the rack to the frame.
I'm surprized at Extracycle's response to your enquiry. Maybe the Extracycle kit doesn't have the threaded mounting hole like the BD frame does.
Oh yeah, and hauling an adult or two kids is no problem.