I suppose you could define a curl wrapper that lets you review the file before passing it along to sh...

    curl_review() {
        result=`curl "$@"`
        echo "$result" > `tty`
        echo "Enter to proceed, Ctrl-C to abort:" > `tty`
        read
        echo "$result"
    }

Then just change curl to curl_review:

    curl_review get.pow.cx | sh

Of course this kind of defeats the purpose of these easy installation tricks.