I just added a feature to tmux-git that makes the tmux clipboard propagate up into higher clipboards for nested tmux. https://github.com/tmux/tmux/commit/4ede35c48c089abe3fca0d94...

Keep in mind that your terminfo has to support this, so make sure your ncurses-term package is the latest release from Jan 28.

I also made a patch for st to support OSC 52 propagation into Xorg, but it was rejected by the st maintainers because it uses base64 (as the OSC de-facto spec from xterm dictates).

The patch for st is however in a branch of the Wayland-fork of st

https://github.com/michaelforney/st/tree/selection

Another option if you use nested copies of tmux is to use this feature to get the remote clipboard into the local one, then yank with xclip.