That's sort of what we do, too, though the numbers are in increments of [BOD×(STR+X)]kg where X is the same number as the canon rules.
For example, say you have an average Human (Body 3, Strength 3). He can carry up to 24kg without any noticable effect. Anything over 24kg and up to 39kg causes the equivalence of a permanent Light wound for the duration of the encumbrance. From 39kg to 54kg causes a Moderate wound, and 54kg to 69kg causes a Serious wound. The character wouldn't be able to carry anything over 69kg more than a step or two (assuming he succeeded in lifting it).
The numbers aren't much different from the core rules until you start getting into the obscenely built or strong characters. And for my personal suspension of disbelief, that's the way it should be, especially in a game of this nature. A troll should almost never have a problem carrying stuff around; they're
supposed to be ungodly strong, and I don't have a problem with a powerful troll at their RML max carrying up to 330kg of weight (though anything over 165kg causes a virtual wound). At least with these rules, a troll
can carry another troll.

Fatigue occurs as per the standard rules.