This sounds almost exactly like the test mail server I setup for our developers. Essentially, any email that hits this box gets dumped into a local mailbox which they can access via SquirrelMail. The primary SMTP servers (that all of our internal processes send email to for relaying) send anything with "@testing.domain.com" to this test box and our development web environment uses the test email server as it's default.
So anything at the development level will allow the devs to use whatever email addresses they want without actually sending email to the outside world and anything above that level they can simply use @testing.domain.com and it'l land in the same place.
This is almost what you're asking for, but rather than piping it to a local mailbox, you could just send the email to a piped alias instead.