I agree with you that it would be "made up" content, but I don't know how else to fill in the missing data. The area not scanned by LiDAR is just upstream from and directly beneath a set of whitewater rapids.

I can guesstimate the shape of the bottom by the behaviour of the flow, and hand-model the missing parts of the mesh. I thought outsourcing that to a generative model would be a nice shortcut -- and who knows, likely it'll synthesize it more true-to-nature than I would.