Python Forum

Just a thought, bear in mind I'm not familiar with YUN, but could you not install the Firmata IDE, pyfirmata and pyserial on a larger computer (Windows, Linux, etc), then transfer the file to the YUN? That would enable you to use a more complete version of Python (including "pip").  From your description, it sounds as if the Python on the YUN is not there for development purposes, but more as a requirement for other programs on the YUN.

As I said, just thinking out loud.

I don't know this distribution. Only 15 MiB sounds interesting. Maybe you can find a way to crosscompile pyserial on your normal PC and repack with the compiled binary for pyserial the image. Maybe they point in their wiki to a documentation how to do this. But it's nothing out of the box.