I was wondering the same thing today. See if this hack works for you. Tmux sets two environment variables inside the session: TMUX and TMUX_PANE. I used TMUX_PANE, but I think either would work. Powerline supports config overrides via environment variables. Taking advantage of TMUX_PANE, POWERLINE_CONFIG_OVERRIDES, and a custom shell theme (without the hostname segment), I was able to throw this in my .bashrc file:

# Change theme if inside TMUX
if [[ -n "${TMUX_PANE}" ]]
then
  export POWERLINE_CONFIG_OVERRIDES='ext.shell.theme=tmux_default'
fi

Change 'tmux_default' to whatever you name your custom theme and put this section before your call to powerline-daemon. Might need to tweak it a bit if you're not using bash.

@weavage Thanks for the solution! I guess it would work for me! Nevertheless I still hope that there's an option to enable this is the segment settings :)