Actually, form what I've read, I don't think MIDI 2 requires any (or very many) changes to Portmidi as Portmidi wraps the various OS-level MIDI APIs and gives you raw bytes. The actual MIDI protocol interpretation is handled in the Pd core.
MIDI 2 is basically an extension of the MIDI 1 protocol and MIDI 2 messages can contained embedded MIDI 1 messages. There is an additional query communication where a device can ask about the capabilities of another device. Overall, it seems to be designed to work seamlessly with older MIDI 1 devices/software.
Short answer is: I don't think anyone is doing this, but it can probably been done by modifying MIDI handling in s_midi.c.