Normally the Pi should switch to composite if the HDMI port is not connected, but you can force PAL by adding sdtv_mode=2 to the config.txt configuration file.
Assuming your composite cable is correct and you've switched the TV input to the RCA input, you should get an image.
Powerblock works fine on Retropie/Raspbian. Open/LibreELEC are embedded systems so if it doesnt work OOTB, it is generaly a pain in the ass to make it work. The support for it should be given by them if they are interested.