The solution was found here: http://joerg.li/hobby/macmini.html
but I'll summarize the important part, the missing link if you will.
In the Password field you can enter three different formats:
- an arbitrary string, e.g. please let me in
- double-quotes + 5 or 13 characters + double-quotes, e.g. "apple"
- dollar-sign + 10 or 26 hex digits, e.g. $5544332211