Well, this goes into what demesnes are, doesn’t it?
Basically, a demesne is an area where Essence accumulates in the world above the normal background radiation. Creation is saturated in Essence, to the point where it is far more common to worry about too much than too little. This energy is moved around via dragon lines, basically streams of energy across the landscape, and a demesne is an area where that energy pools because two or more dragon lines of the same type overlap.
So a demesne is basically a ‘natural’ (as much as anything can be) result of just having way too much energy for the local environment to use effectively. Elemental Essence is usually used up by Creation to do stuff like make water wet or fire burn or allow people to breathe and so on. If you have more Water essence than water then you get a demesne, dig?
So a Yozi Essence demesne would be the same thing. They are places where there is an overabundance of Essence produced by a Yozi. So Malfean essence would be a location where Malfeas is generating more Essence than he is using at that time.
And since Essence is Essence and begs to be used, you can take that overabundance of Essence and put a Manse on it and now have a magical infrastructure to support… whatever you want.