I would imagine some sort of spell effect that gives X points of Feather and of Damage Speed. Then X would be set to the lesser of (Current Encumbrance - Max Encumbrance) or Speed.
As I've learned from a lot of experience in the weight room, sometimes one more dinner plate really is one plate too many. ;^)
Yes, my own weight room experience have also taught me the dangers of "one more plate", though that's a bit different.
This also gives me another idea. Using the same mechanics that the game use to calculate advancements in Athletics, set a condition that keeps track of "steps taken" while encumbered by >50%, >75% and >90%, for example. Then accumulate a number of points, 1 per step at >50%, 2 per step at >75% and 3 per step >90%, when the point reach a certain benchmark, say 1500, then you get 1 point of feather and the total resets. This way the more you're able to carry then the more you're able to carry. It could even be a birthsign, The Pack Rat.
IMO it should be Fallout 3 style, once you're over-encumbered you can only walk.
I think I lucked out in my searc of Oblivion mods, I actually found an Encumbrance mod with a Fallout setting.