Released version 119 with support for plugins. My private install of calibre now transparently strips the DRM from my purchased MOBI files
I haven't documented how to create plugins as yet, but the code is well commented in src/calibre/customize/__init__.py and src/calibre/customize/builtins.py
Just create a .py plugin name it something_plugin.py and add it to a zip file. You can then use the config dialog or the calibre-customize command to add the plugin to your calibre installation.
Right now there is support for creating plugins to perform arbitrary transformations by file type on import into the database, just before a any2* tool is run and just after a any2* tool is run.
In addition all the metadata reading/writing code has been plugin-ized.