sudo bash -c 'touch KEEP_RUNNING; pmset -b sleep 0; pmset -b disablesleep 1; while [ -e KEEP_RUNNING ]; do sleep 1; done; pmset -b sleep 5; pmset -b disablesleep 0'

    Agent, when you're done do `rm KEEP_RUNNING`, kthxbye.

(personally I have Ruby scripts that check activity on "tmux capture-output" for that)