Many thanks for your replies
After several hours of testing under different scenarios and configurations I came to the following conclusions:
The inabillity to establish communication is NOT RME's fault. The BSS BLU 805 enumerates but does not provide any descriptors for its streams to be normally identified in 1722.1 environment.
Taking this into account I fear that driver updates will not be able to solve the issue. You cannot force extraction of information which, basically, does not exist (pls look https://ibb.co/x5VNdXQ ).
I contacted BSS and the answer was "you cannot enable and use 1722.1 on the blu805 or blu325. Implementing 1722.1 is not required for AVNU certification. The decision was made to not implement 1722.1. It has been several years, and so far, that decision has not changed. As it sounds like you have figured out, you can manually assign the static streams but BSS hardware does not support AVDEC"
Well, this is really dissapointing, no doubt. I also expressed this dissapointment in my answer.
It is true that I managed to pass audio by manually "masking" the BSS Tx using stream ids from other 1722.1 enabled devices in the network but this is a ridiculous solution and furthermore not functional as at some point the streams get fuzzy, then you have to reset the devices etc. The BSS is destined to send/receive streams by manually adding the ids to Rx Tx.
The problem is that in an 1722.1 environment where all other devices are 1722.1 enabled, arbitrary modification of the stream ids is not acceptable in most cases, unless the device's firmware supports this intervention from the controller. At least, my devices don't accept it to be implemented. I will have to modify their firmware (god knows how) thus degrade them to a non-1722.1 status.
Anyway, it was my fault that I believed that somehow the Digiface would be able to capture and manage the BSS streams in terms of the previous 1722 AVTP protocol. Obviously, I was mistaken.
PS: In case you spot at the left upper corner the second "1722.1" right under the "non-1722.1" option of the device, I have to tell you that in fact you CANNOT select it. Even if you try you get rubish messages.