I'm quite new to Freestyle but I can imagine using it for great many things. One feature I could really use would be to finetune line thickness with vertex weights, for example masking areas where lines should be visible or just tweaking the thickness of specific sides.
Looking at the current modifiers, closest thing I can find is the Material modifier, but working with textures brings an unnecessary layer of complexity, I feel. And Vertex Colors (if they can even be used for this purpose) are limited to three channels, whereas there can be an arbitrary number of weights.
The python source (parameter_editor.py) looks like it could be used to cut something together to fit the bill, but before dwelling too deep into it I thought I'd ask if anyone knew if it's something that could be done in the first place, or am I running towards a dead end?
Can't answer my own question, so edit.
It seems my timing was rather poor as Freestyle is undergoing a number of tweaks and fixes that might provide tools for what I was after.
Support for vertex groups in feature edge selection and line stylization.
Some hours were also spent for code reading to investigate different options for better Python scripting integration into the Freestyle interactive GUI in Blender. The idea is to allow artists to write Python expressions to build custom line styles.