Here a Reaper project as a proof of concept that with Pianoteq in fact MPE Midi can be used to create mictotonal composing.
->
www.schnellphoto.de/microtonal.rpp
You need to set the Pitchbend range in Pianoteq to -50 / +50. With the demo version this setting is lost when closing and needs to be done again with any start of the project.
Notes are held correctly until the appropriate note-off event is received when new (even very similar) notes are started by note-on on other channels.
Also you can use the hold pedal to hold microtonal notes as if they all had their own strings.
It seems that the Pitchbend in Pianoteq is not perfectly fine. In the example I use 8 steps per semitone (12.5 cent) but there are audible differences in the step size.
have fun !
-Michael