There is a script to scrobble to last.fm from cplay already:
http://sebi.tla.ro/cplay_scrobbler
It uses python-scrobbler for which there is a patch to make it possible to change the scrobble uri:
http://sourceforge.net/tracker/?func=detail&aid=2789702&group_id=207796&atid=1003125
After applying the patch in ~/-cplayrc just change the line
scrobbler.login(USERNAME, PASSWORD, True)
to
scrobbler.login(USERNAME, PASSWORD, True, ('cpl', '1.0'), 'http://turtle.libre.fm/')
(please dont use 'tst' as client code like the patch does as it is only for testing purposes)