As many requested a bulletin board or whatsoever, I first have finished the one I had in work for some time (but priorities did shift all the way along):
Clan Mail
What it can do: you can send to all your comrades (copy to yourself) a mail. That's it.
Same restrictions as you have with normal mails, YET it costs a few gold pieces (currently 10*clan members) to make it less attracting to spam a big clan all over with mails.
Only Leaders or above (founders) can use this feature. If there is a need that officers or above need it also, just let me know.
Personal Allowances would cost more time to program and I don't see a real good point in it... you won't let a low member post mails to all members, and high members should have it anyway.