Actually, IDLE does have code to look for an on-disk copy of the html-formatted Python documentation set but the paths are platform-specific and, in the Linux case, are out-of-date for some distributions at least.  For Linux platforms it looks for `index.html` in either `/var/www/html/python` or `/usr/share/doc/python-docs-x.y/Doc/`.  On current Debian systems, for example, the Python doc package is installed in `/usr/share/doc/pythonx.y-doc/html`.  If you install the doc packages and then create a link, IDLE should find the docs off-line when you select `Python Docs` from the `Help` manual.  For example, for Python 3.3 you *could* do:

    sudo aptitude install python3.3-doc
    sudo mkdir -p /var/www/html/
    sudo ln -s /usr/share/doc/python3.3-doc/html python

That said, the default locations should be updated. And perhaps a more useful customization would be to add a user configuration option for where to look for the on-disk copy of the docs rather than adding another hot key. (Also, older versions of Python are in security fix mode only.)