From http://stackoverflow.com/questions/6510673/in-screen-how-do-i-send-a-command-to-all-virtual-terminal-windows-within-a-sing we have
C-a :                            # (colon) Enter command line mode.
at "#" stuff "my command\n"      # send to all windows
See http://aperiodic.net/screen/commands:at and note that you need the double quotes to enable "\n" to be turned into a newline (single quotes will send a literal backslash followed by an n).