Ubbe, I think the CL of the faction can send a faction-wide message once per hour. Other than that, I am unaware of any faction-wide chat system in the 2d client. I don't know if the KR client supports a faction-wide chat system. Party chat might work for you and does help with keeping life bars up for healing and rezzing, but I know it's difficult to keep it going when characters come and go a lot.
I have a 25-person Teamspeak arrangement (and UOAM) that I set up a few months ago for the Atlantic faction event so our little 7-person guild could communicate more easily with the other people in the same faction with us. Unfortunately, we haven't really used it much. PM if you want me to get you the contact information to set up something like that for yourself and the people you play with.
Edited to add: You can set up Teamspeak so that you assign user-specific names and passwords and therefore have more control over who can get into your Teamspeak. Or you can set it up with a password that someone can potentially pass on to others and then you might end up with complete strangers in your Teamspeak. You can also give trusted people the ability to move people out of specific channels or even completely out of Teamspeak and/or the ability to ban users by IP address. You can also set up channels for groups of people to accommodate separate conversations so you don't have everyone in one channel trying to talk over each other. The program also supports a certain amount of text-based chat or notifications. Ventrillo also offers the same kind of capabilities.