Who invented using ports to circumvent only having 16 channels?

The use of ports to provide multiple sets of 16 Channels is part of the USB support for MIDI transport, so you need to explore the history of the use of USB by MIDI.

I've seen a hint that Yamaha might have been involved, they made a very early USB driver which allowed use of (I think) 6 ports each with 16 Channels, the first one mirroring the 'normal' setup provided by the DIN connectors.

Of course, the whole chain needs to support USB and ports to achieve anything, from the initial creation of th midi data, thru the transport system, any software involved and finally any devices to play the data and make the 'noises'.

Geoff

I looked up the USB midi standard, could not find anything about meta messages of type 0x21:

v1 1999:
https://web.archive.org/web/20150426221331/http://www.usb.org/developers/docs/devclass_docs/midi10.pdf
v2 2020:
https://www.usb.org/sites/default/files/USB%20MIDI%20v2_0.pdf

Looks like Cakewalk was the first user of this meta-event:

http://midi.teragonaudio.com/tech/midifile/obsolete.htm