winelight wrote:I have opted for a single byte, value 0x01. The documentation is most emphatic that this is exactly the MIDI message that will be sent, and only this.
I think the problem is that 0x01 isn't, in itself a valid MIDI message. Hauptwerk will indeed send exactly the MIDI message you specify, but if it doesn't form a valid MIDI message then the MIDI driver/interface/hardware it's sent to will probably ignore it (on the assumption that it's corrupted data), or conceivably even misbehave in other ways.
Any message you specify does need to be valid within the MIDI standard. E.g. you could send a short custom MIDI sys-ex message of your own. Sys-ex is the part of the MIDI standard that's designed to allow you to do custom things. Or perhaps you could send a MIDI control change (or NRPN) for an unused MIDI controller number.
winelight wrote:Instead, what I appear to get sent over MIDI is the following SysEx message:
F0
7D
02
F7
That's Hauptwerk's standard 'MIDI feedback test' sys-ex message, which it always sends to all enabled MIDI OUT ports, to check them for feedback loops. Just ignore that message.