It is technically possible to listen in on and send chat messages to some of the public channels, but also some of the public channels are location specific, so you won't be able to see them if you're logged in via Chat Only or if you logged your bot character off in a location where that channel doesn't exist. I don't remember what channel is what in all honesty. From memory, OOC channels are Global, but shopping channels are zone specific. If you are in one of the major cities, the shopping channels are linked (so you can see OT Shopping from Omni-Ent, Omni-Trade, Omni-HQ for example, but when you leave the city the shopping channels are zone specific.)
Anyway, when BeBot was created we intentionally didn't make it possible to listen in or send chat to the public chat channels in order to raise the bar on anyone wishing to create a spam/ad bot and similar. It can be done, and all chat clients will recive chat from the channels when connected to the games chat servers. FunCom implemented all channel filtering on the client side. So even the official AO client gets everything going to Clan OOC, Clan Shopping, etc. all the time, even if you have that channel muted in preferences.
if you know how to write the code to do so. Posting to the public chat channels using a bot is also a good way to get your account banned by FunCom.
Assuming that your intent is to allow new characters to to talk to your existing organization, you could use the bot's guest channel to provide this, though to use any kind of access control on this you'll have to know the character name in advance, have the character name communicated to someone who can add guests, leave the chat channel open to everyone, or wait for the character XML to refresh to have data for the new character.
If your intention is otherwise, such as recruiting members for your organization, or advertising your organization, etc. this would again go into EULA violation and would end up in getting hit with the ban hammer.
Lastly, I don't recall if the newbie channels are instance specific. I don't think they are, but you wouldn't be able to access the new player area unless you have your bot character camped there, and if you camp your bot character in the new player area you won't have access to other channels (don't recall which at the moment though.)