Just got my FCU last week and was looking for something like this but found nothing so I did a bit of digging with the in-game dev mode, looking at the Behavior vars and I was able to get the FCU fully functional with displaying the lcd digits as well as the state led for AP modes:
CRS -> OBS setting. Index of 1 or 2. Course CRS
SPEED -> (A:AUTOPILOT AIRSPEED HOLD VAR,Knots)
HDG -> (A:AUTOPILOT HEADING LOCK DIR,Degrees)
ALT -> (A:AUTOPILOT ALTITUDE LOCK VAR:1,Feet)
VS -> (A:AUTOPILOT VERTICAL HOLD VAR,Feet/minute)
I could share my profile to make it easier but not sure if that's allowed.
Edit: for some reason, the ALT does not update while in ALT HOLD, I need to press either VS or IAS and then will change as I select new values with the knob, maybe a BS dev can shine some light? It's not a major issue but it bugs me a little as it's so close to perfection.