One thing that maybe worth trying is setting the clock on your PC back to a date prior to the subscription expiration date and try playing a few songs without being connected to the internet. If the PC is not online, Windows has no way of telling if the time change is to correct a wrong time or deliberate.
I am not sure what Windows Media Player does when the expiration comes. If it modifies the song itself, then it may still be possible to play songs that you haven't tried playing after the expiration date if you set the clock back.
However, if Windows Media Player logs that the licenses have expired in its license database, then there is pretty much no hope in reactivating them by setting the clock back. Much like setting the clock back on expired shareware software.
Finally, if you are willing to resume your Napster subscription, then chances are that your music will re-activate once your subscription is live.