I can't say specifically what the situation with the Seaboard is, but it is possible to apply poliphonic expression with traditional MIDI, albeit with a major c

I can't say specifically what the situation with the Seaboard is, but it is possible to apply poliphonic expression with traditional MIDI, albeit with a major caveat. For example, the QuNexus keyboard that I have here can do it. "MIDI Poliphonic Expression" (MPE) essentially involves only playing one note per channel, which means that things like pitch-bend (which apply to every note on a given channel) can therefore be applied per note. The major downside of doing this with traditional MIDI is you're still limited to 16 channels, but with MPE it then also means you're limited to 16 notes.