It should be fairly straightforward to measure the cadence in the way you describe, but you would still need a way to measure input speed. Startona uses a physics simulation to calculate the player's speed based on the power input since this gives the most accurate results. In case power is not available, the game uses speed as a fallback. Although, it would be fun to add a way to connect an Arduino-based solution via USB so people can homebrew their own speed measuring systems (with hall sensors or similar). I am open to adding that option if enough people ask for it.
You could try to use a Bluetooth speed sensor like the "Wahoo RPM Speed Sensor" if your home trainer somehow allows you to fasten it to it's flywheel - if not, you would still have the option to put your bike on a cheap roller. That way the game would work, but you would only get a limited experience since only the high-end rollers support erg mode. At that point, you have come full circle and it may be cheaper to opt for an entry-level hub trainer like an Elite Suito.