Does anyone know of an effective way to determine, using http://www.creationkit.com/Condition_Functions, when the player is currently jumping or in the air?
I can't find anything to check for specific animations like the jumping animation, and simply checking for Z velocity leaves an inevitable gap at the height of the jump if I want to avoid false positives while running on steep hills.
Specifically, I'm trying to negate damage while the player is jumping backward.

I doubt you'll find a conditional that specifically does what you want... I did a quick check but didn't see anything.

But given your objective, you could use the GetMovementDirection conditional, combined with an zPos check. So you'd set your LogEntry conditional to the GetMoveDir, then the fragment would check for zPos changes (to preclude walking backwards triggering your thing).