When I check F9 it's usually about 4 of every 5 players using + already... and I don't think it would completely break the GIP list for non-plussers.

As for the 'No plus' events, you could use them to move non-plussers into Text events telling them the level is to be played with JJ2+ and where they can get it. I'm sure not every new level will be playable only with plus, but if there ever come plus-only events that do special things normal JJ2 can't do, I think a simple event parameter can be very useful to block the level (or provide alternative functionality) for non-plussers. If the instant kill system with CtF bases can't be fixed (or if something else turns out to be broken and can't be fixed) this could provide very good alternatives too.

One more idea:
-Event "Wallclimbing" with boolean parameter "On": if set to 0, wallclimbing will be switched off if you hit this event; if set to 1, wallclimbing will be switched on if you hit this event. Very useful for GroundForce levels as you can disallow wallclimbing after the players have warped into the waiting room so they won't 'cheat'. Of course, the /wc command should set an initial value and should not be able to override the event.