FOP2 does nothing to allow or not allow things in Meetme. It justs sends AMI commands to mute/unmute. It has one catch, depending on your asterisk version and manager configuration, sometimes the initial mute state that can be set "outside" fop2 is not acknowledge, this happens mostly with confbridge, but it might happen with meetme. In that case, you need to mute "twice" from fop2 to actually send the mute command, and to let unmute work.
However, back to your post, you DO NOT HAVE TO USE THE "s" OPTION !! If you do not want users to be able to use a menu from within meetme, DO NOT USE that option. Again, it is not a fop2 thing, it's an asterisk dialplan configuration thing.
FOP2 does not change any configuration in Asterisk for anything, it justs fires up commands, and read events. Whatever happens after that is asterisk and not fop2. Try it with a custom conference where there is no "s" option, then *X will not work, fop2 or not fop2.
Best regards,