Pretty sure John Symms was throwing them at pipe comps back in the days. For some reason, I have a memory of him throwing a flat 7 or 9 at a pipe grand prix in like 2008. It's kind of weird that nobody are throwing some at pipe comps.
There is a few people that do them Flat 7's that is, but I think this year I only saw maybe 1 person do it in competition, not sure who it was, some small name. But I remember Sean Field use to do semi off axis alley-oop 9's which were about as close to flat 9 in the pipe as I have ever seen.
that would be an extremely odd rotation and axis. ive seen a snowboarder do fs rodeo 7 or 9 down the pipe once and it was crazy looking. but never have i seen someone do bs rodeo down the pipe above 540.