I have heard of this happening (albeit in very limited circumstances) and find it a little bizarre. Would anyone accept paying extra should the fares rise after booking and paying?

To answer (or rather not answer) I'm not aware of any current 'deals' along these lines (not to say there aren't any). It would be nice, but not something I would be expecting.

Once I've booked and paid for a fare I tend not to check back later, to avoid disappointment.