I will say, I prefer public grid to be open. If a scene needs to be private or limited, take it to TP or RP or private rooms. Sure, a flag on +who or whatever would be nice, indicating a hangout or location is 'closed' scene, but really there are places to take locked/closed/private RP.
I do like the idea, if you ask for RP (hey wanna rp, or on chan, or whatever), you set the scene. I'm just not so sure about asking if someone wants to RP, and following up with, 'what do you want to do?' If you initiate in any way, have some ideas at the ready. Wiki's are a good tool to get an idea of a character you're asking for RP. Or be forward ... I'm looking for RP, don't know what do to, and am open to suggestions. Just avoid the 'what do you want to do?' ... and you won't get the 'I don't know, what do you want to do?" reply.
Edit: I do miss those days, wandering a grid to find someone then posing whatever came to mind, or seeing what was going on and jumping in.