Assumption correct Tony - the Roadster gauges are CANBUS so many of the signals into it (incl the speed and rpm) are digitally encoded.
I have not tried the Roadster but on other Triumph Keihin ECU CANBUS models of that generation, the bike will not start without the gauges being connected.
Can someone perhaps test/validate that?
(If the bike WILL start without the gauges being connected, it would still be possible to connect after-market non-CANBUS gauges, but wiring would be different (and not everything available at the Main Harness Instrument Connector Interface)