Someone still needs to create the torrent, distribute it (and/or the magnet link) and seed the content (that is the distfile) until there are enough users seeding it. I may very well be wrong here, but I also believe that two users creating torrents with the same content can end up with different torrent IDs/checksums/whatever. If that is the case, you would need to clearly define who is to create the torrents, and putting that responsibility on the port maintainers is not going to be popular. Having a centralized service which updates the port collection, downloads the new distfiles, creates the torrents and then seeds seems to be like the only remotely reasonable way to implement this, especially since it to my understanding could be implemented without modifying the port files.
There is another P2P network called eDonkey2000 (not sure about correct casing etc) or in short ed2k which is possibly more suited for this, from a technical perspective. The problem is that the ed2k network is very uncontrolled and filled with stuff you don't want anywhere near your computers or networks. The advantage is that you can never end up with multiple different torrents for the same file, meaning that all downloaders could potentially be served by all uploaders sharing that file.
[edit]
I forgot to mention something, there might be licensing problems for some ports as well. While probably quite rare if even existing, care needs to be taken to respect such licensing restrictions if an automated system is to be implemented.
Also, since distfiles are not FreeBSD specific, the entire open source community could greatly benefit from a service like this. Perhaps one even exists already? In that case, adding support for FreeBSD ports to use that as a download source is much more realistic.