It seems like what you're really asking for is not a programmatic /command to control join/part displays, but you want individual channel controls. Or perhaps you're just asking for both.

In any case, yes, individual channel controls for hiding join/parts would be nice. However, I'm not sure how high on the list this would really be, given how easy it is to script:

Code:
on ^*:JOIN:%hide.channels:haltdef
on ^*:PART:%hide.channels:haltdef
alias hidejoins %hide.channels = $addtok(%hide.channels,$1,44)
alias unhidejoins %hide.channels = $remtok(%hide.channels,$1,0,44)
alias listjoins echo -a * Hiding join/parts on: %hide.channels


You would /hidejoins #mychan to hide it, /unhidejoins #mychan to unhide it, and /listjoins to list what channels the script is working for. Note that I've ignored the QUIT event since the purpose isn't to write you a script but rather to point you in the right direction. It's not much harder, but it's not as braindead simple as JOIN/PART.

Again: yes, it would be more convenient to have a channel specific option for, but there are lots of more immediate issues that need dealing with.