Lugged carbon frame... Is it safe?

Surely the force going into the rear triangle is the same as if you just had a brake on it? And since a decent brake can lock the wheel instantly into a skid, and that doesn't break the frame, why would there be any problems doing it with your legs?

Not really, the brake is in a different position to the chain, so the forces applied to the rear triangle will be different when using a fixed wheels are opposed to a rim brake.