Chaz, if you restrict any mode through ircd (let's say +G), the services server, which is u:lined of course, will be able to remove it.
So any channel owner can simply do "/cs set #chan mlock -G" or "/cs clear #chan modes", and bypass the restriction this way, after the services server will send "-G" rawmode.
He can even set mlock off again after the removal.
I assume that if a network owner restricts a mode through ircd, it should be restricted from services too, and won't consider as functionality kill.
Therefore, I suggest a config directive named "CSRestrictMlock" (or "CSRestrictMlockClear"\"CSRestrictModes") which will provide a restriction from anope's side.
There's the known mlock notice: "Mode r ignored because you can't lock it", so I guess that some part of the method is already implemented.