As far as I am aware, the only way to get it done is to post a thread at the .Mac iDisk support forums requesting it and then waiting for a forum admin to respond, but as I say, I haven't done it yet myself as I'm too a) lazy and b) OK enough with the workaround.
As it isn't a universal issue, my suspicion is that people who were using iDisk prior to the WebDAV switch are those who were most likely to suffer this problem... I don't know if that is true though.