John – I did a lot of structured experimentation as you call it and nothing helped so far.

I played loud, I played soft. I played with a pucker, I played with a smile. I pushed from below, I played fast air, I played slow air, I raised my tongue, I lowered my tongue, I played with ‘taa’, ‘tuu’, ‘tee’, ‘too’, etc.
I pressed the MP hard into my teeth, I didn’t as well (I don’t any longer).
I set my MP more on the top lip, I set my MP more on the bottom lip, etc.
I experimented a lot.

I CAN FEEL where the higher notes ARE! They simply don’t speak to me. That’s it. The middle of my aperture doesn’t vibrate at higher frequencies.

John – one thing you obviously can’t accept is that your experience could be totally different from others and what is MOST important of all – YOUR trumpet playing POTENTIAL can be different from anyone else’s potential.

Maybe for you the transition to start playing above the staff was simply one step away. Imagine someone who is 10 steps away from that range or even 100 steps away.
It’s not simply taking those 100 steps – it’s the ability to retain what was achieved.

Steve asks: “What should I do to play trumpet higher?”. I don’t think he has to do anything special. He has been playing trumpet for 20 years. That means he had enough time to figure out how to play higher.
Let me repeat – there are hundreds of kids who with the worst technique can play up to high C. That shows their potential: they can fix all the bad habits and play up there much easier.

I wish I would be wrong! – Time will tell.
(Sorry Steve, do you need another 20 years to figure it out? Do I need another 10 years to figure it out?)