I am looking for resources, documentation and general advices on writing a virtual MIDI device (see my previous question for reasons) and basics of generating MIDI events from a VST plugin.
The target platform is Windows, but Mac compatibility would be a plus.
I hope you don't mind me editing the subject of your question -- but I think that you shouldn't be so worried about the VST part of your software here, and the nature of your question is more about driver development than VST development.
That said, you are essentially trying to write a normal MIDI driver, but you don't actually need to connect to any hardware. So writing a virtual driver is really not as hard as you anticipate... you just need to find the driver API's (see the edit to my answer on your previous question).
One more thing I should add here, is that the best way to accomplish what you are trying to use a separate application with Rewire. So that is to say, you write an application which initializes a MIDI device when started, and connects to the sequencer via Rewire. Your users would start up the sequencer first, then your application, then send the audio on a bus track to your app, and configure their sequencer to receive MIDI from the device driver which you would write. The app, conversely, streams audio from rewire, does some type of FFT to get the pitches (or whatever you want to do to the audio stream), and then pushes those events out to the host through the MIDI driver API. IMO, that's probably the best way to solve this problem here, if I understand your project correctly.
Couldn't you just use existing virtual Midi devices such as Midi Yoke?