There should be no difficulty at all in doing this. If they dial into
your network then they use your outgoing mail relay server, and yours
alone. Period. (Unless you have some kind of agreement in a roaming
system where you authenticate your own users to someone else's dial-up
and vice versa, in which case you only allow the user to connect to the
the "home" ISP's mail relay host(s).)
Or for those who complain that "reconfiguration" to use the local
mail relay is too difficult, howabout setting aside a /24 from
somewhere (the swamp ?) and writing an RFC that says "thou shall
not route this" ala private address space, but if you wish to offer
"local" services, then the user only need caryy one "well known"
address range around with them.
Give it some well know DNS: "mail.local." 
and so on. (OK New
TLD required).
This could also fix deciding which DNS servers to use amongst other
things.
Peter