Maybe a special mouthpiece with a little valve at the stem. If you start to use too much pressure, the valve closes.
Actually, hasn't there been an invention like this before (ah, searching through the land of my vague memories)?
Other ways to reduce mouthpiece pressure (the "high blood pressure" of brass playing).
- While playing a medium-high note, slowly take the trombone off the face. Eventually, the note will break up. How much can you lower the pressure before losing the note?
- While holding that same note (or higher?) have a friend stand behind you and, unannounced, gently push the trombone off your face.
- Stand with your back to the wall and play ascending lip slurs. The wall should prevent you from pushing in.