I doubt its ever been done
The way the two systems work is different
Everything on the 500 is networked, and goes into the dash via 2 wires and is the translated by the dash computer, assuming the fiat circuit diagrams are right
On the Panda most is still networked but some of the module signals are feed in to the dash via separate wires
Although the plugs look the same on the 500 only about half the plug is used on the panda most of the plug is used
I am assuming the arbarth is similar to the rest of the 500 range as I don't have it's specific diagram
The reason some tj can be grafted in and work with the original dash is because the engine CAN data from there ECU is in the same formate as that from the pandas ECU, i.e. fiat used the same network protocols