Frozen Wasteland: Midichlorian
Great, this module implements exactly what I need here, and it works indeed nicely as documented when used in monophonic mode. For example, I used it with MTS-ESP set to 22 tone equal temperament (to have a tuning that clearly deviates from 12 tone equal temperament), and got a plain VCO and, e.g., the Dexed VST in HOST (set to a pitch bend range of 1 semitones up and down, as required by Midichlorian) nicely in tune.
A MIDI monitor VST in HOST shows that indeed there is a pitch bend message sent to the plugin for every new tone.
However, when used in polyphonic mode, the HOST module still shows the problem I described initially: a MIDI monitor shows that the VST plugin then receives only very few pitch bend messages, most are somehow lost, and the sonic results are not in tune, even when playing monophonically in poly mode.
Of course, the limitations of the MIDI protocol (only one pitch bend for all notes of a MIDI channel) are partly to blame for this problem, and it might be that the implementors of HOST therefore did not even bother trying to get pitch bend working properly for polyphonic mode. A clean solution could be to add MPE support to HOST, then all MPE-supporting plugins could work nicely with polyphonic pitchbend etc., but seemingly that is not available yet (see also this discussion).
Summary: I can now monophonically play VST plugins in HOST with arbitrary microtonal pitches (e.g., from MTS-ESP) by separating the pitchbend component from a microtonal V/OCT signal with the module Midichlorian. Unfortunately, this does not work in polyphonic mode (even though for some reason the Midichlorian documentation claims to output “polyphonc Pitch Bend output for Host”).
Thanks a lot again!