After more testing, I managed analyzing the generated midi file with a midi monitor in my DAW, ableton Live. it happened that Ableton deleted all the multichannel features of all tracks, in my case, the instrument 29 was finally always played on channel 1, even if OpenMPT generated events for channels 0 and 3 for this instrument. What I suspect, is that ableton has a very strict midi importing rules which are : midi type 0 is always one track, maybe multichannel, while midi type 1 can be multitrack, but never multichannel.
This situation makes it impossible to have a one shot export/import setting if I don't have a intermediate tool (like MidiEditor), when any module has any instrument playing on several channels simultaneously.
Maybe it can be an interesting subject to debate about among the dev team, but what I suggest would simply be to have an option disallowing OpenMPT to generate multichannel midi tracks, which would be coherent with what I've read about the midi standard. On this particular case, Ableton Live (and surely some other DAWS) would be very happy to get additionnal tracks