[mod redacted: Please review the rules before posting]
Björn... Thanks for that. I understand SIP and disabling it so you can use software from unlicensed developers. I would also expect that the StealthPedal should work under MacOS 10.12 as the handling of kernel extensions didn't change. But this is not an issue for me as I am happy to leave my 'studio' iMac at 10.10 anyway. Maybe when I upgrade it things will change. Its a 2009 20" but I do have a 2013 21.5" with a dead PSU I am repairing that might end up in that position. And yes, I have SP MIDI Control 1.1.1 now but, as you have said, only after directly asking support for it. Why it is not available in the user area is beyond me.
So guys, your input has been quite helpful and here's my latest news on this saga...
I've got it working!
1) Erased the drive (yes, I backed everything up first)
2) Installed a fresh 10.10.5
3) Installed SP MIDI Control 1.1.1
IT WORKS! (SP MIDI Control shows signal and the control light stays lit)
4) Installed AT 4.0.2 - no SP MIDI signal
5) Installed AT 3.1 - SP MIDI works
I then went out for a few hours and came back to a 'sleeping' computer. After waking it, the MIDI had failed again. I am so annoyed by this. Then I recalled someone (within this thread) talking about the need to keep the SP MIDI Control app window front-most. I launched AT 4.0.2 again and assigned the SP to a stomp. I then brought the SP MIDI Control app to the front and what do you know, it's working. As soon as the app is sent behind another window, it stops working in AT4 however I can still see activity in the SP MIDI Control app. Strange.
I don't know what the problem was to start with but I do know I have a lot of software re-installation to do now. I'll be very careful and test the SP after each item is installed, just in case the issue was another piece of software conflicting with the SP control app. Also worth noting that I haven't tested this from with a DAW, only stand-alone, so I'm yet to find out if the plug-in is going to play nicely as well.
Stay tuned!