I was actually lurking here to find an answer to a problem I was having related to this.
If you switch to custom MX, the "non-editable" webmail.yourdomain.com record appears to be removed and you can add a CNAME or an A record of that name. I haven't verified that it resolves since that was not what I actually wanted to accomplish.
I was looking to create webmail.yourdomain.com as a subdomain and that fails with "Can't add domain: already in our DNS system."
Haven't dug for a workaround yet. If I don't find one I'll probably open a support ticket.