Yeah, so far I have not been able to do this either.
It is a shame, it something I loved about fhem.
Also fhem could detect IT codes by itself since it listened to the received signals.
I guess, we could setup the nanoCUL as a serial device binding, and use Regex to detect certain codes?
In the changes, I did on the CUL transport, I added some trace information. You can see all serial data, which is sent to and received from the CUL. The other day, I wanted to add a new remote and thought I could use the trace data to determine the code. But no data was received by the CUL or rather sent over serial connection.
As I can see so far, no X21 command is send to the CUL to provide receiving information. There is no implementation to handle such information, either.
Maybe, I can implement something like that, if I find some time. But I think a OH2 version of the binding should be developed than. There are some mechanics to create items, when a radio packet is received, and so on.