Writing a custom scene exporter for Blender turns out to be ridiculously easy compared to just about every other 3d package I've come across, mainly due to the power of Python introspection. This Python script shows how easy it is to introspect over a Blender Scene. After some fiddling I ended up with the Python script at the bottom of the post, which dumps the entire scene as an S-Expression for a Lisp instance to read.
My Lisp-based export problems are over!