Graeme,
thanks for sharing that video, this is of course a great way to visualize U - the relative flow velocity to the line. You can also take two neighboring markers and draw in the normal vector to the fly line, this should look similar.
The video demonstrates that the underlying assumption of this thread is false because air is not blowing on the inclined section of line in the rod leg.
No it's not wrong, it's just an simplified example; U is the relative flow velocity. Note that you don't need a horizontal flow to generate a vertical force component (lift). E.g. I'd expect for the point marked with a red circle in #248 an upward force component as a result of form drag and skin friction.
You just can't apply the illustration in my first posting directly to video analysis, a inclination does not mean lift force. The goal was first to quantify the resulting lift force for certain cases to get an impression if it's significant and I've made a table for some cases.
For video analysis we need to generalize the model for a fly line segment first, that it works for all kinds of movements, like rotations too (step (3) in the roadmap). I have already a model for that, but need to check first before I'll write about it. With position and velocities of two markers you can compute then the resulting forces due to form drag and skin friction.
I've meant with step (2) verification of fly line segments. That means we measure the form drag and the skin friction of a piece of fly line and compute realistic values for the drag coefficients. If that's not possible we should at least do some plausibility checks and compare literature values. Maybe someone has access to a wind tunnel?
With (3) we can do video analysis, this requires a dataset of positions and velocities of every marker on the fly line. Ideally the markers should be closely spaced. With that we can compute direction and magnitude of the resulting forces of form drag / skin friction.
Maybe is the roadmap pedantic, but I think that's better than doing hand-waving and we need patience, because doing these steps require some time.