Lets continue this pattern command extension discussion in other thread, shall we? It is becoming totally off topic in here.
So lets assume we get the pattern command id extension since it is really neccessary and continue from there. Otherwise this whole discussion is pretty pointless, because then only implementation would be altering 9xx behaviour if marker mode is on.
Do we want to automate playback behaviour at marker positions (notecut, noteoff, continue, loops) or keep it fixed? If former, needeth is two commands or dual command and restricted amount of marker points (not with extended command precision), even though I doubt more than 16 would be needed especially if we have 9xx offset coupled with Mxy. Unless they would be used for some bizzarre workaround for some even more bizzarre problem (got any?). If latter, it would be best imho to offset 9xx inside marker range if coupled with M-command, M0 in pan/vol or M00 in command colum for last used. Otherwise 9xx would behave as normal. Just as Johann said.