As above it wouldn’t work. Also remember that you may be a customer of Onto for a while, but you may not keep one specific car long term. Even if you don’t decide to swap cars, it isn’t unheard of for them to contact you and say they want to swap you into another vehicle (even if it is for an identical car) to clear out some of the older cars in the fleet for example. If that happpened you would either forefit the registration or give Onto the headache of trying to move things around to keep it for you, add it to fleet insurance etc.
Think of Onto as a long term car hire. That’s essentially what it is, and with very similar limitations too. No real modifications like that would be permitted since it isn’t your car, and the next person to hire would be expecting to receive it in it’s original state.