QuickTime apparently has a built-in MIDI synthesizer. I would like to send MIDI output to a virtual MIDI device that will use this synthesizer. Can this be done?
Quicktime 7 had Midi playback capabilities, but no method to generate its own data for output that I'm aware of.
It didn't sound very good anyway & almost no-one ever used it for anything even half serious ;)
Quicktime 7 is deprecated & will no longer work in Catalina. Newer Quicktime versions do not have Midi playback capability.
In Applications/Utilities/AudioMIDI Setup there is a routing device known as the IAC Bus [Window menu or Cmd/2 to show it]that can route Midi between any capable apps. It's always been somewhat under-used & tbh even as a sound engineer who's been using Midi for 35 years I haven't had occasion to use it since maybe the early 90s.. honestly, I've forgotten how you even wire it up these days.
I'm sure you'd be better off finding a cheap or freeware Midi sequencer app - Garageband comes with the Mac, but there are others.