It's always worth a bash trying anything which may improve safety, but I can't help but feel that selective attention will come into play with this type of system. Most of us have unconsciously trained ourselves to ignore messages which are not preceded with our callsign. Any overly wordy messages (beloved by us Brits), will also have a negative effect.
If the marker beacon is only powered when the red stop bars are illuminated, then a simple "Stop! Stop! Stop!" would certainly get attention and perhaps produce the desired result most consistently.