I don't think the 8.5 cents mech fee would apply at all if the number of plays is being tracked. It is either a purchase or a performance - not both at once.

Tracking plays could be done within the playback device as Brian described and then uploaded to the server the next time it connects. That is how DirecTV does it. Your sat receiver phones home once a month and reports what you bought on PPV that month. (Hackers will probably figure out how to circumvent this, but posession of a hacked iPod would be just as illegal as possesion of a hacked DirecTV authorization card.) If the device doesn't get a chance to "check in" when past your subscription expiration date it would presumably stop playing.

Since it is just performances in that case all payments could be distributed through the PROs which is a good thing because a new mechanism doesn't have to be invented and the PROs are already set up to track all the sub-dollar amounts.