Never satisfied, I built the VST3 version again and tried that with no dice. The VST2 version works exactly as expected, but VST3 remains silent - I presume because it is not generating any MIDI output. I plugged a MIDI Monitor effect into the Halion instrument and that is what I observed: messages flowing through when using the VST2, and no messages when using the VST3.
I’ve read a lot (both here and at Steinberg’s forum) about how MIDI is treated differently between VST2 and VST3, but haven’t come across anything that specifically states this isn’t possible - in fact quite the opposite, such as here:
…which specifically discusses MIDI CC events, whereas I’m currently only concerned with the simpler note events.
Any thoughts on what I may be missing that means no MIDI events are being output from my VST3 version?
UPDATE
The answer was already here right on this forum. My issue (noted in my first post) was that I’d started with the Arpeggiator tutorial. This example expressly omits all audio inputs, which are apparently needed for processBlock() to be called in a VST3. The specific answer I’m referring to was here from @stephenk (thank you! - worked first time):
I am now a very happy camper. I’ll let this community know if/when I finalize something useful. Thanks again to everyone I’ve quoted above.