The tmux server machine needs an ~/.ssh/rc with this line in it
ln -sf $SSH_AUTH_SOCK ~/.ssh/ssh_auth_sock
The tmux server machine also needs a tmux config with the following line
# Use a symlink to look up SSH authentication
setenv -g SSH_AUTH_SOCK $HOME/.ssh/ssh_auth_sock
Then when you change the ssh auth socket, you can run this to update your current shell
eval $(tmux show-env -s |grep '^SSH_')