ginseng,
I don't know about OpenBSD, but on FreeBSD boxen you can use the pw(8) command with the -h flag to pass it a filehandle. Using pw usermod <username> -h 0 will allow you to pass the password to STDIN. However, to keep it off STDIN, you could create another filehandle, snag it's FD#, and pass it through that FH.

Hope that helps, lots more info in the pw(8) man page.

-marius