This is the kind of thing that has helped me the most too. You have essentially discovered what F. M. Alexander discovered in the late 1800's. Although very logical, it is not an easy thing to discover on your own IMHO. (To learn more about the "Alexander Technique" goto http://www.alexandertechnique.com/)
For me, the key to relieving tension and opening up the throat is just
what you've described. That is, in order to relieve the tension,
you must be able to know what it feels like. Sounds simple but, as we go
through life we assume postures and actions that become so habitual that
we cannot identify them. For example, if you tense up each and every
time you play high, you do not know what it feels like to relax and play
high. Therefore, you tense up every time you play high. This
is the basis of the Alexander Technique; Identify the tension, then control
the tension, then eliminate the unnecessary tension. For me the key
was to try to consciously create the tension and then relieve it, away
from the horn. Tension in the high ranges is, I believe, fostered
by the assumption that playing high is difficult,
so, we must try harder.
The way I now spell "playing high" is "m", "e", "n", "t", "a", "l". IMHO, the best thing you can tell yourself is that it is *not* difficult to play high. It is difficult to learn because we often get in our own way, but not really very difficult to do. You must try to make it as easy as possible. We have the advantage when playing trumpet because we already know what it feels like to play with ease. Play a middle C. Most players feel very comfortable and at ease playing this note. Now, extend the feel of ease and lack of tension to the notes above. Once you start feeling the tension, go back down and get the feel of no tension. Then try again. Let the sound be your guide and do anything and everything that you have to do to keep the same full, rich sound and still play easy. That is, tongue level, lip compression (lip to lip), air flow (more than you think you need), oral cavity size, etc. If you don't use enough air on the lower notes, by the time you get to the next higher note you are starving your embouchure even more. Then the higher you go, the worse this insufficiency gets. This begets tension very easily.
One difficulty that I think you have addressed very well in your approach is that we often cannot control a single muscle group tension without inducing it in another group. For example, we must using tension/compression in our corners as we go up. For me, I found that this induces more and more tension in the throat also. It is being able to separate these muscle groups that I think is often a difficult step. That is, to be able to tense the corners while keeping all the othe muscles relaxed. For me, one great way to do this is to practice it off the horn. This is, again, an Alexander concept of gaining control over the tension and realizing what it feels like to eliminate it.
Many of us have had the experience when playing that, all of a sudden we find that a note we though was hard, came out easily. Then we try to figure out what we did to make it so. I find that rather, it is better to concentrate on how it felt when you did it. Then, try to reproduce that feel. For me this happens when the tension is low and the air flow is high.
Each time you try to play higher than you can in a relaxed manner, tension will creep in (it does for me). The key is to try to extend the feel of relaxation higher each time you play. At some point, however, you may end up extending the tension into the lower range if you try playing high too much. Then it is time to put the horn down for a while. The key is to get rid of the Pavlovian reponse of high notes=tension. As many have said, to play high you have to play (practice playing) high. However, IMO it should be practice that incorporates the entire range (top to bottom) constantly. Each time you go down, you reinforce the feel of relaxation.
All of the above is predicated on the assumption that you are not using mpc pressure to get the high notes. If you are, then it is necessary to get all the other aspects of your embouchure moving too (as described above) so that you can get rid of the pressure. Mpc pressure is a high note killer just as tension is. Not to mention a chops killer.
As always this is just what has helped me and is my opinion. Comments are welcomed as long as you are gentle. :-)