I actually thought it was already possible, by assigning a negative item weight to the armor item, but this approach also triggers when players just hold it in their inventory, as opposed to wearing it.
I think it could allow for some interesting meta changing in modpacks, since then you'd also have to consider that lighter armor would allow you to carry more.

Note: I hope this mod isn't dead, I'm attempting to make a zombie apocalypse modpack and it fits perfectly!