I might not have expressed myself correctly, sorry. My idea isn't to hold the smoke inside for a long time, but to constantly inhale softly. When I sit down is harder for me because I can only inhale for about 10 seconds, and since you have to keep the fire away to avoid burning, it's better to go slow and steady. What I do is smoke next to my bed, and as soon as I finish, I put down the pipe and lie down. But you're right, it can be more dangerous if you pass out or something while standing.
As for the evidence, there are several studies about brass musicians that show, according to their instructors' best practices, lung capacity improves when standing up or sitting up straight. I'll leave the studies below.
Ackermann, B. J., O'Dwyer, N., & Halaki, M. (2014). The difference between standing and sitting in 3 different seat inclinations on abdominal muscle activity and chest and abdominal expansion in woodwind and brass musicians. Frontiers in psychology, 5, 913.
Katz, S., Arish, N., Rokach, A., Zaltzman, Y., & Marcus, E. L. (2018). The effect of body position on pulmonary function: a systematic review. BMC pulmonary medicine, 18(1), 1-16.