Looking at the PR511 it seems like one can not sent out a code that is being used by the PR511 for turning it on-off.
That is correct. The Leviton 6417 I mentioned sends out HailAcknowledge on its own house/unit code when it sees motion, but the PR511 doesn't do that. (I just verified with my PR511.)
However, you can use one of the SENSOR switches to send a command to a neighbouring unit code. For example, if your PR511 units are set to A5 and you set the SENSOR/+1 switch to IN on all of them, any time one of them is triggered by motion, it will send A6 ON and, once the TIME DELAY has elapsed, it will send A6 OFF. You could create a macro in AHP that is triggered by A6 ON and sends A5 ON, which would turn on the floodlights on all PR511 units. It is important that you also create a macro that is triggered by A6 OFF and sends A5 OFF, or all of your lights will be left on indefinitely.
One thing I found out with my Leviton 6417s is that if you send an OFF command to its address, it won't trigger on any motion after that until the TIME DELAY elapses. I just tested this with my PR511 and it doesn't seem to do that, so hopefully you won't have that problem.
The Leviton 6417 was a more elegant solution for the need. Too bad none of these are made anymore.