There is a mini-mod for Stainless Steel that models supply. It's called Byg's Grim Reality. It doesn't model attrition, but gives traits reducing morale, based on the length of time a unit is away from it's base.
Its rather similar to the siege traits you get in Europa Barbarorum I. It is, however, a bit more extensive. As far as I've seen it works well. Maybe an alternative to proper attrition?