Hi Stan,
I have been checking the A1 FAQ's and Manuals but you are correct, there is no information about the possibility to use the USB C port as an analogue line in connection.
From what I have read
here in the old forum, it looks like it would not.
Maybe you would need to add a USB adapter with ADC but IMHO it would add clutter to the system, while I would not be 100% sure it would work either...
Long story short, it would be easier to use a speaker with an analogue line in connection, such as A1 1Gen, Beolit or Beoplay S3 (well sounding but not portable, though!), as I did for my sister.
They can be found 2nd hand and they are not very expensive.
link and
here some examples of the results from other Beoworlders.
Another option is to connect the TT to a BT Streamer (NOT a Receiver!) that it can be easily hidden.
Then the streamer would send the BT audio signal wirelessly to the A1, that can be placed anywhere in the room.
There are many BT streamer options out in the online forest...
I understand it is not the "yes, it can be done" type of answer you were hoping for but still it might be of some help.
Regards.
Olivier
EDIT: ADC (Analogue to Digital Converter) instead of DAC (Digital to Analogue Converter).