I realize this wasn't the question, but:

amanda (www.amanda.org, see pkgsrc/sysutils) works well on NetBSD, and
does dumps to holding disk and then streams it to tape.  It's a bit
complicated to set up, but I'd set it up even for a single machine.
Once set up it's very low hassle - in my group at BBN we have 3 amanda
configs backing up ~135GB in ~170 filesystems.