python-serial is indeed pure python and works on OS2008. I have used it, and dbus, to connect to my roomba and other projects over bluetooth without much issue. You may want to incorporate turning the bluetooth on and off as well into your project, which can be done over dbus.

download python-serial from the source forge page listed above, use an archive program to unpack the contents, and then just copy the folder from the archive's ./usr/lib/python2.5/site-packages/serial to the same path in your file structure.