Hi Andy,
las week I did exactly that. The recipe is:
1) create proper directory in sd card
cd /mnt/card
mkdir python
mkdir python/bin
mkdir python/lib
2) get the python packages from the web, and download them to the sd card
3) extract files to proper location:
for each package you want to install,
3.1) extract the package in a temp directory
ar x python-coreXXXXXXXXX
tar xvf data.tar.gz
3.2) check what files have been extracted, and to which directories, and move them accordingly to the previosly created directories
i.e.
move usr/bin/python /mnt/card/python/bin
NOTE: if your SD card is formatted as fat (as mine), then symlinks cannot be created in the card, and you'll receive lots of errors telling so. To solve this issue, you must write down what symlinks could not be created, and, instead of creating them, just copy the origin file over and over. It will take much more card space, but it's the only way and, anyway, its space in the SD card
4) create a symlink for pythin executable
cd /usr/bin
ln -s /mnt/card/python/bin/python
5) to run an app, just go tho the script directory and run
./py.py
This setup works for me. Hope that helps.