'copy-pipe' mode command to copy selection and pipe the selection to a command.

This is exactly the functionality I was looking for back when it didn't exist. It wasn't immediately obvious how to use this, but here's how:

bind-key -t vi-copy 'y' copy-pipe "redis-cli -x set status"

set-option -g status-right '#(redis-cli --raw get status)'

And now I have a message I just copied to redis in my tmux status bar. I'm planning on doing this with clipboard text next.