I would do it in a soap or xmlrpc wrappers, over an already well made, very lean, http server library. So then the clients could be made in any language without having to implement a client for the strange protocol we would be implementing.
I dont think speed or network load is an issue here. I mean, i dont see deployments where a bit of latency in changing passwords, adding users...etc. would be an issue.
If you have a 30 million users mailfarm, then your bw and iron is having a hard time in smtp or imap, really, a bit of extra traffic in thease kinds of transactions is not important.