The question here is how to pass an arbitrary set of parameters to
freetype from fontconfig. I'm not sure what will be the most natural
on the fontconfig side (they already support a number of "complex"
configuration value types, such as a 2x2 matrix). I'm thinking that
for simplicity we may want to restrict "module" configuration to
polymorphic lists, not trees. Anyway, I'll ask the fontconfig people
for advice.
I'm certainly no expert in that area, but I know of users of freetype
(think small embedded) that would likely not want to have a new
library dependency to get AA rendering. However, it certainly makes
sense to modularize this part of freetype, but maybe it should still
belong to freetype (the project), even if it designed in modules that
can be loaded/unloaded at runtime (for instance).