Hi @highsiderr 
I have researched on your suggested model of multitimbral usage of OsTirus, and now I am able to arm a multichannel layout with just an engine instance, driven by further MIDI chains thanks to the ingenious internal rooting of Midi data devised by @jofemodo and @riban.
I must say that for me this is already a huge step forward, on the path of the real-world OsTIrus usability on the Zynth. 
Nevertheless, what remains to be ironed out, for my specific use scenario, is how to select a Multi patch saved on VNC from an external DAW, which for me is important because I use Zynthian primarily as a multi-channel Midi receiver, for externally arranged compositions.
In a nutshell, what I aim at for the time being - until a complete integration of the plugin is achieved - is not to change presets within a Multi on the Zynthian, but to be able to recall consistently a saved Multi from a DAW, for the Z to play the Midi flow that I am sending to it (while also potentially receiving individual program and parameter changes, on different Midi channels of the same Multi).
I have investigated on the topic, on the basis of the original hardware’s documentation and other online knowledge, and so far I have come up with the following workflow, for selecting a Multi patch within a specific user-saved bank:
1] Instruct the synth to switch from Single to Multi mode, by sending:
CC122 as on/off (presumably 1/0) toggle
and/or a sysex message with the HEX syntax
F0 00 20 33 01 dd 72 00 7A vv F7
where dd addresses the Device ID and vv the Midi mode (0, 1, 2).
2] Dispatch a bank select command to the plugin, by using CC0 (MSB) and CC32 (LSB).
3] Point at a specific Multi preset in the selected Multi bank, with a Program Change message.
Notwithstanding, I did not manage to make it work until now, and OsTIrus does not respond to my externally sent Midi configuration commands.
As far as I can see, these could be the possible issues:
A) My Multi patches selection procedure is simply mistaken.
B) OsTIrus does not receive Midi config messages because they aren’t sent to its Midi Global Channel, but I have not been able to find a related selector in the Common and Menu areas of the plugin’s GUI on VNC. For the moment, I am surmising that it might be Ch 1.
C) OsTIrus does not receive Midi config messages because the Midi IN port is not correctly set, as either Port 0 or f_Midi, however the engine loaded on Zynthian appears to receive Midi messages through both ports, with a reassuring blinking “m” symbol.
I will maybe try to report the problem upstream, unless @dsp56300 shows up here to offer some direct insight!
The ultimate goal would be to reliably recall and control Multi patches on Osirus and OsTirus, from an external computer or sequencer, switching on the VNC server only for designing Single presets and building Multi layouts.
Best regards
