Plenty of softsynths can do this now thanks to MPE, so the idea of a midi input vocal harmonizer with MPE abilities (specifically single-note pitch bend) would be amazing.

Antares Harmony Engine sounds best but it's not MPE and seems to use it's own midi build, so pitch bending individual notes doesn't work. Even in Reaper where you can manipulate incoming midi messages.

BLEASS Voices claims to be the first MPE harmonizer and would be exactly what I'm looking for, but it doesn't have pitch-bend parameters; let alone individual note pitch-bend ones. You can set the MPE 'slide' function to midi fine-tune pitch, but its scale is -100% to 100%. Instead of a percentage I'm looking for specific parameters like "Pitch-bend +/-2".

I compose in experimental musical tunings so I use a Reaper based script that manipulates incoming midi information via pitch-bend. It's super powerful (when it works) and can fine-tune any midi note to two decimal points, but it NEEDS the instrument to have specific pitch-bend parameters. Which almost all MPE instruments do... except for BLEASS Voices lol.

Any idea if something like this is out there, or maybe how to build one?

 

Have you tried using Fluid Pitch for this? I have never tried to do what you are trying to do. My first guess is that you are asking for some pretty specific features, and you will end up needing some sort of workaround. You won't find everything addressed by just one plugin. Hopefully you can.

Reaktor Blocks might be a good place to start.