> Nifty 4.4BSD trick: union mount ~ftp/etc on top of /etc, and then just 
> do a normal pwd_mkdb.  You may get a complaint about entries missing 
> from the password file when you go to unmount it, but it works just 
> fine.

I just tried that.  It *is* nifty.  Note that you'll have some empty
directories to clean out when you unmount it.  I still think pwd_mkdb
should be more flexible, but at least there's a _cool_ workaround.