Can I add my .02?
The physical characteristics of a 'free' buzz, and the process of producing tone on a brass instrument are hardly related... IMHO (and there are some dissertations that provide some good foundation for this)
First of all, consider the aperature - buzzing (there needs to be a better term) on the instrument confines and focuses the aperature. Then, with the instrument on the face... does it provide a system of high and lower pressure chambers, one on each side of the lips. The process of equalizing the pressure (when you blow through pursed lips) forces the air stream into vibration - from deep inside the chest to the termination of the wave (end of the bell). I seriously suggest looking into Daniel Bachelders thesis on tone production on the trombone. He put a fiber optic camera in the mouth piece and observed the buzz... comparing his video to the 'free' buzz shows a striking difference in approach, production and result of this process.
I have found (for me) that a loose buzz helps get the blood moving in my face... but other than that the buzz itself does not seem to be a reliable medium that will improve my playing ability... or any others for that matter.
Combinations of activity with lips buzz might help ability... I have found that singing a pitch, and then matching it with a buzz without stopping the air in-between has helped mine and my student aural acuity... but still I have yet to find any real reason to buzz. This activity is more of a pitch matching exercise than anything else.
It seems to me that more important than a good buzz off the instrument... is a good steady flow of air. And with the constriction of the muscles in the face and neck - to be able to focus the buzz enough off the instrument could cause undue tension in the air stream when you add the instrument.
I would much rather work on a focused air stream with my instrument - than a fat buzz off my instrument.