Você pode usar a run-shell
opção , mas o O mais importante é separar os comandos com \;
Neste caso, algo como:
bind R source-file ~/.tmux.conf \; run-shell "echo 'Reload'"
run-shell shell-command
(alias: run)
Execute shell-command in the background without creating a window. After it finishes, any output to stdout is displayed in copy mode. If the command doesn't return success, the exit status is also displayed.