The final determination about whether a specific part is or is not legal for use on a competition Robot must be made by the inspectors and Head Referees at an event. Teams must be prepared to remove any component of their Robot that is deemed illegal by the inspector or Head Referee, including decorations.
That said, we can provide some general guidance on the examples you provided.
Would it be considered functional for teams to 3D print custom motor caps / casings using a filament that changes colour based on temperature to provide drivers with visual feedback on motors overheating?
Generally speaking, a custom 3D printed piece that is attached to a motor to provide visual feedback about that motor's temperature would be legal under clause G of rule <R9>, which reads, "Decorations which provide visual feedback to Drive Team Members (e.g., decorative lighting) are permitted, provided that they do not violate any other rules and serve no other function (e.g., structural support).
Would the fact this motor cap fully encases a motor be seen as providing the motor with extra support / insulation?
If the 3D printed part provides any structural support it would no longer qualify as a non-functional decoration and would be illegal for use on a competition robot. Fully encasing a motor is likely to be considered extra support. A good rule of thumb to apply would be to imagine whether or not the motor would remain intact with the 3D printed part attached and the assembly screws not included. We believe insulation, if any is provided by this 3D printed part, would not give the Robot a competitive advantage.
If the above is considered non-functional, would it be considered functional for teams to not screw in motor screws to doubly use these casings as "Hot-swappable motor caps" to easily change cartridges without the need to unscrew motors, or would this additional functionality now make it a functional component?
This would be a clearly functional use of the 3D printed part, and would not be legal in the VEX V5 Robotics Competition under any circumstances.
