Lanefan
Victoria Rules
Hmmmmm...interesting. That's sparked me to think about how an exhaustion (or starvation, disease, etc.) mechanic might also affect one's casting capabilities....you can be starving in a desert suffering from a diseases but you won't see any of that affect the amount of spell slots you have.
Easy enough to make functional in a system that uses spell points rather than slots; there, each defined degree of exhaustion might simply knock off 10%* of your spell points (as in 10% of your starting total, i.e. if your maximum is 30 spell points each degree of exhaustion would knock off 3 regardless whether you happen to be at 27 s.p. or 4 s.p. at the time).
With a slot system it would be very difficult to codify exactly which slots are lost at what degree of exhaustion at different levels; even worse if those slots are tied to prepared or pre-memorized spells.
The only way I can see this working with slots without getting way too complex in play is to have the effect occur later: if you get exhausted today it affects how many slots you have (or can prepare) tomorrow e.g. each degree of exhaustion yesterday costs you one slot per spell level today, or something like that.
With something like this in place one could also invent poisons that attack spell capabilities.
* - or whatever percentage makes sense, based on the granularity of the exhaustion mechanic.

