The docs specifically say that it's possible to load additional plugins on a per-project basis by editing the .exvim file:
Load Vim-plugin on demand for different projects based on your .exvim settings.
However, I still haven't found how to do that. I've thoroughly scanned the docs, and even looked at the source code of relevant exvim plugins. Hints?
These .exvim files are not regular vim files, so I can't just put Plugin 'foo'.
This should be documented somewhere. Related: #58.