This turned out to be a three pipe problem; all I wanted to do was not to run OpenBSD's send mail daemon, yet still be able to send mail from that box. It turns out that there's a simple answer in the post Improving Sendmail Security by Turning it Off, by Hal Pomeranz. From the post:

Note that you can also simply hack the submit.cf file provided by your vendor. Just look for the line that reads D{MTAHost}[127.0.0.1] Just replace [127.0.0.1] with the name of your new relay host. For example, D{MTAHost}mailhost