Context: I've tried HCE in combination with reader mode and these two work like a charm.
I'm looking to provide some sense of backward compatibility with the reader mode app.
Is there a way to read or send NDEF messages over Beam from an Android 4.3 device to a reader mode enabled application?
No, the whole point of the reader mode API (enableReaderMode()) is to disable peer-to-peer mode operation and activate only NFC reader/writer mode. As Android Beam is based on peer-to-peer mode (NFCIP1 + LLCP + SNEP) there is no way to receive or send Beam messages on a device where reader mode is active.