@wxl Not sure exactly what your use case is, but you might have a look at vipe(1) if you don't know about it. It's in the moreutils package on my system.
command1 | vipe | command2
» vipe allows you to run your editor in the middle of a unix pipeline and edit the data that is being piped between programs.
I wonder if it's more a function of the shell you're using than the actual stdin vs stdout type stuff.
Does ZSH/tmux/screen offer a way to mung output after that command has already been executed?
I'd also suggest sh or csh but... I have a feeling they likely won't be capable of what you're looking for.