Telecom models don’t allow for this. Demarcs are ports. One side owns the port, the other side owns the cable that plugs into it.

If only someone standardised a router layout with customer-owned and provider-owned modules and a neutral, state-owned, bridge hardware. Similar to how TVs deal with conditional access. And no, SFP ports are not a solution to this problem, because they only have 1 plug each side.