A little update. It seems that reading out RSSI values is possible after all in OOK continuous mode. I haven't conducted thorough tests yet, but it seems that until the RSSI threshold is reached by an incoming signal, the RSSI reg. just returns some arbitrary constant. Then it starts changing according to the RSSI it currently senses and stays that way. If I set the threshold to the highest sensitivity value then values start changing right away. I've written a few lines of code that spouts RSSI values in continuous OOK mode and logged a session into a file with RealTerm, and loaded the file in GoldWawe (not the best app to look at waveforms of this kind, but it's OK). Attached image is a screenshot showing a capture of the old, cheap 433.92 MHz I use for testing sometimes. It's actually a floor higher, and several thick walls away.
This of course opens up new possibilities for the OOK receiver. It becomes possible to receive and decode OOK signals without HW modifications to the Moteino, or in general, less pins need to be connected and free to use on an ATMega. Natural consequence of the ability to measure RSSI is the possibility to adapt the receiver to the signal strenght to achieve more accurate pulse length detection, and so on. Of course,
someone else has already done something similar, and actually his work gave me the inspiration to dig into this more. I however want a more generally applicable simple lib for basic OOK reception for custom codes and experimentation, and not another variation of a multi-device all-inclusive receiver solution.