In tmux 2.9a, man tmux says (under attach-session):
-r signifies the client is read-only (only keys bound to the
detach-client or switch-client commands have any effect)
Based on this, you can make a shortcut to go read-only (and back) by providing your own binding to :switch-client -r:
# Toggle read-only with <prefix>R
bind-key R switch-...