Add the preview to see what you're actually stashing:

   git ls-files -m -o --exclude-standard | fzf -m --print0 --preview "git diff {1}" | ....

And that's just the start: it could even be that by binding a key to the fzf reload command to then display the diff in it's finder, and in turn a key to stage the selected line, you could turn that into an interactive git staging tool.