Spanky's suggest will surely work, but will flood your PL with x-10 traffic.
My solution was to use a PCS dimmer which has a scene dedicated to a 50/50 (1 sec on, 1 sec off) flash rate. One x-10 signal and it will flash until you signal it to turn off. In fact I use this as a visual indicator that the alarm has armed when I leave the home...flash for 6 seconds and turn off.