Is there a way to have a generic modifier that takes as input a custom piece of python and the mesh data coming from the previous modifier in the stack and, as output, new mesh data for the next modifier in the stack?
If its just for prototyping, you can put the first half of the modifiers on one object, then use that as input to set vertices on a copy of that object using a frame change handler or whatever. Its not the fastest thing in the world, but its easy enough to set up.
It’s not currently possible to write modifiers in Python, but as @Jack_Simpson says there are some other ways to manipulate meshes.
Well, that’s unfortunate, sometime would be useful to have the mesh data coming from another modifier instead of the object, like when there is the need of using a skeleton or the subdivision modifier in a non destructive way