Dear PreSonus, I submitted a support ticket request and received the following answer from the PreSonus Technical Support Rep:
"You are correct, there is currently no way to enter a device ID.
The reason for the request is that it is the recommended option for stable most accurate clock synchronisation, to use an external clock device providing MTC to the DAW (ie. to Studio One 5), which is consistent with the option in the Studio One 5 Advanced menu for enabling slaving to an external clock device ("Sync to External Devices"). This part of the process can be successfully accomplished by then selecting the external clock source in the Midi Time Code drop down menu (ie. in my case selecting the external midi interface port dedicated to this function - eg. Motu Midi Express XT Sync In-Out All port). This results in Studio One successfully locking to the external device clock via MTC (noting that the external device may need to be configured to map its MTC stream to the computer running Studio One); the result is Studio One displays eg "Running - 25fps" next to its MTC menu in the advanced options menu and starting/stopping the external device transport control (ie. pressing play/stop on the external device) results in Studio One 5 starting/stopping its transport controls and displaying its timecode clock synchronised to the timecode clock of the external device - all good so far.
Unfortunately, this also results in the external device overriding being able to use Studio One 5's transport controls (either by clicking on the computer screen within Studio One or via the PreSonus FaderPort 8) - pressing eg play/pause/stop on the FaderPort 8 causes a momentary effect but is immediately overriden by Studio One 5 syncing back to the external device clock - Studio One immediately reverts to continuing locked on to the external device's transport control/timecode.
The recommended way for synchronising a studio's devices/DAW is using such an external clock as above, which is more accurate and stable than the clock coming from the computer - the process requires the DAW (ie. Studio One 5) to be able to send MMC data back to the external clock device addressed to that device's MMC Device ID (in this case, the Motu Midi Express MMC Device ID is "19") so that Studio One 5's own transport controls (both on screen/clickable and when pressed on the FaderPort 8) control the external clock device's transport controls. That way, the external device acts as a central accurate stable time clock hub to which all other studio devices can sync, including Studio One 5, and be controlled by the Studio One 5 transport controls.
While Studio One 5's Advanced Options Synchronization menu options has an entry for selecting Midi Machine Control in a drop down menu, this menu seems to be for incoming rather than outgoing MMC data and (therefore) there is no place in Studio One 5 to enter the external device's MMC Device ID such transport control data is to be sent. Note that in the external device it may be necessary to tell that device to receive such MMC data from the computer via, in this case again, the Motu Midi Express XT's Sync In-Out All port, a setting which I expect any such external clock device has nowadays).
Thank you - this is a feature which will be relevant and is necessary for all from the home user to professional user in order to utilise the Studio One 5 Sync To External Devices MTC option at the same time as retaining full transport control functionality within Studio One 5 on screen and with an external transport controller such as the FaderPort8.