For what it’s worth, I think a simple convention that would sidestep the “when does the change happen?” question is to mark the change at the last possible stroke where not making the change will result in a “trapped” pick. So people know their “last chance” to make the turn, and can decide how urgently they want to move over to the exit lane, so to speak.
And notation and terminology-wise, I think the notion of “inward” and “outward” strokes (which I think you said Ben Eller has advocated) would help keep the “trapped/escaped” concept orthogonal to “upstrokes/downstrokes” in the minds of students. “Upward-slanted downstroke” is probably harder for people to keep a mental grip on than “outward downstroke”. In addition to making it more clear that the magic is in the movement direction of the pick rather than the orientation of the pick in space (which seems to be a pretty frequent source of confusion), the “inward/outward” language also avoids the confusion that arises when some people think “downward slant” ought to mean “tip of the pick pointed toward the floor”, rather than “end of the pick furthest from my body pointed toward the floor”.
If I were to undertake marking up tab for my own use, I’d keep the established “upstroke/downstroke” notation, and supplement with a letter “i” for “instroke” and a letter “o” for outstroke. I’d put them vertically aligned with the existing pickstroke notation; not sure whether it would fit better “above” or “below”. I wouldn’t mark up the whole score though. I’d mark the first note (or maybe 4 notes) of each major section, and if there were any particularly tricky parts, e.g. two-way pickslanting, or basically anywhere I had to do anything “special” to clear a string other than one-way pickslanting, I’d add "i"s and "o"s to mark those up. If there were a lot of recurring motifs, I’d probably only mark each one up the first time it was encountered.
Edit: And one handy aspect of “i” and “o” notation is that it’s super easy to write quickly by hand, especially if you just use an undotted vertical stroke for the “i” and infer that it’s an “i” from context. Mind you, I’m not a big music-reader. Is there a major obvious conflict between this and an existing notational convention? (I mean, simple letter “o” circle should be easy to distinguish from a whole note, but is there another conflict I’m missing?)
Edit 2: And I’d probably use a red pen or something the underline any sections that diverge from one-way slanting, except in cases where that would result in so much red that it became meaningless.
Edit 3: That notation probably covers crosspicking as well, people just need to know enough to infer that if you have a passage with alternating downstrokes and upstrokes that are all “outward” (ending with the pick “escaped”), the way you achieve that is with curved pickstrokes (or stringhopping if the passage is slow enough).