So not an HP meter that linear accumulates damage, but a system for tracking how disrupted the material is at one time? It could be derived from the material properties (some form of MAGICAL_VISCOSITY value token or something) and affected by the overall size of the body part made of the unusual material.
I think what you're referring to is what IRL is called
Reynold's Number. It describes the turbulence of a flow of liquid through pipes and around objects. (Or depending on your perspective, the turbulence created in a fluid when an object moves through it.)
The fun thing about it is that it's straightforward math and physics. It takes in the velocity of an object (or flow of liquid, depending on perspective), a length variable which is dependent on the context, fluid density and fluid viscosity, and outputs a dimensionless number.
And while it's depending on the situation, a good rule of thumb i've learned is that any Reynold's Number >10.000 is likely to be a turbulent flow.
Putting a mechanic like this into Dwarf Fort means making the assumption that any turbulent flow through a gas/liquid beast is damaging to them, and that "killing" such a beast means you're basically stirring them to death.
It also means doing extra math to add some kind of object form factor to the Reynold's equation, since blunt attacks are more likely to create a turbulent flow than, say, stabbing them with an adamantine spear.
And then there's whips.
Since increasing speed increases Reynold's number, and the tip of a whip moves faster than sound, a whip effectively becomes a monster-killing weapon.
EDIT:words.