As for making a MIDI device with Bela, check out the PureData/cv-midi-cv example on the IDE.
As for MPE specifically, I think the advice you got on the PureData facebook group (I assume it was you?) was a good starting point: [poly] gives you an output index that is the "voice number". If you use [poly 15] you'll have 15 voices (as the 15 channels in MPE). Then get the index output of [poly] and use it to address a specific channel. E.g.:
[poly]
| << output is voice, note, velocity
[$2 $3 $1( << reorder the output: note, velocity, voice (channel)
|
[noteout]
if you need to send also other types of channel messages (e.g.: control change), you should keep track of the voice(channel) number and use it in the other midi objects (e.g.: [ctlout])
