Thanks! I didn't know about INTERACTIVE_COMMENTS.

Adding a comment there is a neat trick. It also works with bang searches (!?), almost like creating a temporary ad-hoc alias:

    find . -name "*.png" -print0 | xargs -0 -P8 pngquant --ext .png --force 256 #optimizepng
    ... many other commands, time passes ...
    !?optimizepng