This P4P type local peering can be added to generic BT software. All that is needed is to do a WHOIS for x.x.x.x@whois.arin.net (where x.x.x.x is your IPN) and you get back your local net block. For me this would be NETBLK-OOL-4BLK (NET-67-80-0-0-1) 67.80.0.0-67.87.255.255.
I think the operative words here are "CAN BE". How many will go to that trouble?
That depends. If I were a BitTorrent client publisher, added this support, and advertised this new feature, I can see this capability being used to take the wind out of the sails of the ISPs that claim the P2P uses too much of their Internet Bandwidth. The existence of this feature would be a "meeting the ISP half way" gesture and place the ball into the ISP's court to make a similar gesture (or just shut up and knock off with their anti-P2P rhetoric and shenanigans).
