What exactly do you need to communicate to/from Reaper about?
Do you need to communicate from your Processor?
Do you need to communicate from your Editor? (Very possible via OSC).
[See my babble about OSC] Triggering abelton live clips from juce plugin? - Audio Plugins - JUCE
Look at the Default.ReaperOSC located at C:\Users\yourusername\AppData\Roaming\REAPER\OSC.
It is (sort of) self documenting.
At a minimum you should be able to discern what is available via Reaper / OSC.