The easiest way for starting RPiPlay as soon as the user is logged in is to add it to a user's .bashrc file:

pi@raspberrypi:~$ nano .bashrc

Add this to the end:

# within an SSH-session, ${SSH_TTY} will be set
if [ "x${SSH_TTY}" = "x" ]; then
  /home/pi/rpiplay # Or wherever the rpiplay binary is located
fi

For automatically logging in after booting (and thus automatically starting RPiPlay), use the autologin option in raspi-config's boot settings.

In case you want to stop RPiPlay, all you have to do is then to connect a keyboard to your Pi and press Ctrl + C.