Encrypt home partition

Guesswork from man page. I have not tested this yet!

Locate which partition/slice home is mounted on.

cat /etc/fstab

Configure a softraid device on the located partition/slice.

bioctl -c C -l /dev/sd2e softraid0

Passphrase will be asked for. The same command will be used for remount at a later stage.

Check which the new encrypted device is.

dmesg

Write zeroes to the first megabyte of the encrypted device. Just for initialization the first time.

dd if=/dev/zero of=/dev/rsd3c bs=1m count=1

Delete volume again.

bioctl -d sd2

Reference

Part of installer?

I agree that OpenBSD should keep the installer as simple as it possibly can. But maybe one additional question requesting which partitions to encrypt would be simple and common enough to include.
If you want to try to encrypt the manual way I found a simple guide (via Google). A tiny bit more extensive than the man page.

<a href="http://metasyntax.net/guides/openbsd.pdf">OpenBSD Guide - Encrypted partition</a>
<a href="http://www.openbsd.org/cgi-bin/man.cgi?query=bioctl">bioctl(4)</a>