So, based on lectrick's comments on this previous hint and man perlfaq5, the following script places a random 12-digit-string (letters and numbers) password on the clipboard. It sits there for 60 seconds, and is then overwritten with a random entry from Emacs spook.lines file:
#!/bin/sh
jot -rc 24 48 123 | grep -m12 \w | rs -g 0 12 | pbcopy
sleep 60
perl -0e 'rand($.) < 1 && ($line = $_) while <>;' -e \
'print $line;' /usr/share/emacs/*/etc/spook.lines | pbcopy
Remember to make the script executable (chmod a+x scriptname). When used with Quicksilver or similar utility, this becomes quick indeed. However, don't then don't forget about the stack in the clipboard, which will show the previously-created password.

