Donald, is an interesting issue indeed.
I assume you dealed with possibility of a splitting structure , so I will not write about it (have you?).
Like Szymon explained, the plan is perfect, divided into variable and fixed portions....
So,
other possibility is the one to assign GL expenses directly to an activity type [1] in this way you could differentiate independent activity costs (e.g fixed) vs. dependent activity costs (e.g variable). Of course this means that every transaction has to be recorded, taking in consideration if it's directly related to an activity type or not; foresee the aggregated complexity this could represent into your daily operations.
It would be interesting to read some folks that had tried this method, or share their own experiences regarding the issue at hand.
Br, Humberto