And yeah, definitely have been there with just not logging in very often when I heard people complaining about idleness.
To me, it feels super demanding to have someone criticize people for being idle off grid/in private areas. What does it matter? Maybe just do a +where if the game only supports people in public rooms and stop typing WHO. The thing is, especially these days, it's never not going to be awkward to be the one asking for play or sitting in public. Yes, it would be super nice if when you logged in you saw a selection of people in multiple places that you could choose between, wouldn't that be nice? But I think most game cultures by and large (Arx is an exception there but it's still no guarantee) don't have people sitting public most of the time anymore. So if you want to see people sitting in public you have to do it. Or you might have to be the weirdo to page people and say "Hey, wanna play? I have an idea and a place, if you're interested in meeting up." People will say no a lot, but I mean it's not like you're getting to RP if nobody is in public and asking either?
And yes, I'm aware, somebody has to do it. I just think it's good to keep in mind it can tend to be the same sombodies asking for RP or setting it up, and when that is unacknowledged/unreciprocated/if they don't initiate then there tends to be silence, while in most cases it's probably not personal (though sure some of the time it will be, nobody is everybody's cup of tea), it can still feel kind of lonely. So I think it helps if people do try to step out of their comfort zones just as much as they complain about nobody else doing so.
As far as staff action/encouragement, though--I really can't think of much except for staff STs running stories/plots/scenes. Most incentives get exploited for as long as they last, but I don't think that makes people stay out in public more after that fades (despite the noble intention). I think eliminating ooc areas/private areas just means people won't log in at all, which I guess would reduce annoyance factor for some people but still won't mean more RP is had by those that are there (especially if it requires initiative/people asking). I do think keeping numbers low enough for personal attention helps as well, but that's hard to manage long term especially on a public game. I think it's as much if not more important to have buy in from the players than direction by staff, esp. long term.