I may be wrong and hope to be corrected if I am, but I don’t think too much playing on the leadpipe alone is intended. Some exercises yes, warmup yes but not to be exclusive. How can you know if what you think you are accomplishing translates thru the horn. It takes a pretty good ear to get little things thru the pipe. If you are just playing the leadpipe exclusively I would ask Greg about this, I just don’t see it being productive for less than a very advanced player and then judiciously. You have to get to the feeling of just the leadpipe and then try to replicate that with the horn attached. You have to compare and listen critically to learn from your sound IME. Sorry for the suggestion if this isn’t what your are doing.
Rod