John, Greetings from the Last Frontier
I have found buzzing through the leadpipe a very developmental procedure; however it is not the same as buzzing through a mouthpiece, and both are required for a fuller practice schedule, as well as lip buzzing.
I generally lip buzz to begin with. 2 or 3 scales and maybe a couple in minor or some other mode. Generally I take a tone generator with me. I record myself playing scales on the trumpet and then take the tape with me in the car so I have a solid base from which to work from.
Then I switch to mouthpiece work and I always use a BERP, I know BERP's are designed to be used with the trumpet in hand but they will work alone too(if you're in the car).
Then I go to the leadpipe. The use of the leadpipe is important for developing an open sound and for listening to attacks. (it really let's you know when your attacks are getting sloppy) And as during practice I always make sure that I rest the chops as much as I use the chops during this period. You can plan your rests on the tape. I have found that a straight forward methodical approach for in the car work can eat up an hour quite effectivley.