This is what I am planning on doing. I need to accomplish the same.
ipad POS
utilizing the AIRPLAY feature, stream to AppleTV
AppleTV HDMI output to HDMI to RCA converter
HDMI to RCA converter to BNC connection
BNC connection as input into CCTV DVR.
This should work in theory. I have all the components ordered but we will see if it works after implementation.