Yes, it certainly looks at times on the two bass trombone videos that the tongue is touching the lips for the initial attacks. That said, I don't know that it is for sure. There are times when I'm pretty sure it is, as the upper lip appears to be pushed forward slightly by the tongue, but that might be just the lips getting firmed up at that attack drawing them back in against the teeth. There are times when I suspect it may look as if the tongue is touching the upper lip, but it may be on the teeth behind the lip. As I said above, we don't know exactly where along the "slice" we are looking (or how thick that "slice" is). The lips are curved. If the focus of the fMRI is to the left or right of where the tongue is striking we could be seeing a part of the lips that appears further back behind where the tongue is striking. Does that make sense? I'm not an artist, like Harrison, but imagine an overhead view.
Mostly I'm just trying to qualify to make sure that we admit that we're really speculating with those videos.
In Doug Elliott's embouchure film there was a bass trombonists who absolutely did tongue between the lips, if I recall correctly. You could actually see the tongue tip between the lips at the point of the initial attack.
I would like to go into an fMRI and participate. It sure *feels* like my tongue isn't striking my lips on initial attacks, even on a pedal F.
I couldn't always play that way, it was something I worked on over years.
Dave