It turns out that the rear anchor points are part of the rest of the frame. Here's what it breaks down to without destroying anything:
Closer view of the anchor:
It's fairly easy to get it to that point with two screws holding it to each rail.
Do you want those, or would it be better to just have two complete seat base frames? Cost will be $40ish for two frames (shipping FedEx ground), probably free-ish if you just want the anchor with the cross-bar.