Actually, you can specify the target port in the SRV resource record
(RFC 2052), which is currently Experimental and more or less ignored
by everyone. However, there is a move afoot to put it on standards
track (draft-ietf-dnsind-rfc2052bis-00.txt). If that happens and it
gets widely deployed, then in a few years it might solve your problem.
Yes, but we already have MX. We don't need SRV for email, and we
certainly don't need the disruption that would result from having
yet another way to find the recipient's mail domain.