Removing penalties from Medium Encumbrance

[coyote6]

I'm trying to make the Easy Travel armor property (from Magic Item Compendium) work. The property lets the wearer carry up to a medium load as if it were a light load -- ignoring max Dex, check penalty, speed reduction, etc. (It also lets the wearer walk longer, but that's not a thing that affects the character sheet, so it doesn't need any real implementation.)

What I thought I'd do is check to see if the hero is at medium encumbrance; if so, reset the encumbrance level to light. This is what I put in the eval script:

~ If hero is at medium encumbrance, make it light
if (hero.tagis[Encumbered.Medium] >= 0) then
  herofield[tEncumLev].value = 0
~  herofield[tEncumAbbr].value = 0
endif

It doesn't really work; if I set the timing late enough, it does change the tEncumLev -- but it doesn't affect the max Dex, check penalty, or speed. (Changing the tEncumAbbr values causes it to display as being light encumbrance, but doesn't change the numbers.)

I'm not sure if it's a timing issue or not -- I tried several different stages and values, including PreLevel/10000 & before (which, IIRC, is when armor check penalties are applied), but then nothing works.

So, I suspect that altering tEncumLev (and/or tEncumAbbr) won't work. Unless there's a specific phase & time where it will work, maybe?

Anybody have any ideas on what needs to be changed? And when -- when are the encumbrance-based penalties applied?