If what schema said earlier is true, and that the new chat system is an IRC server, the only way to do this is to have the client individually PM every user within 30m. It could get a little DDOSy if you smush a whole bunch of people in a small space, but that's not actually that big a deal; IRC is pretty lightweight, you'd need a lot of people in a tiny area, and there are better ways to crash servers right now anyway.
For what it's worth, I don't think this is very practical. 30m is really close together given how large sectors are. I guess there's some marginal RP value, but realistically PMing someone directly is probably just fine.
Binding a channel to one sector is actually really easy, since sectors have unique identifiers. Something like #sector-2-2-2 for 2,2,2 or #sector-5--3-7 for 5,-3,7. Ideally there should be a restriction in place that prevents you from simply joining these channels whenever you feel like it; I shouldn't be able to listen to what's going on at someone's home base just because I know where it is and I can join that channel. If I want to listen to their conversations, I should be required to actually go there. Although, realistically, if you're talking about something important, you shouldn't be using any public channel, even if you don't think anyone is listening right now - but that's a different fish.
System channels would be cool too. Galaxy channels might be interesting if people actually went to other galaxies or if it was actually feasible to determine what galaxy you're in based solely on your sector location, but I don't think we can do that.
