A derringer made by Bond Arms, down in Grandbury, TX, has an optional trigger guard.
I fired a derringer-type pistol once that had a manual safety, a springloaded cross bolt device. By pulling back the hammer slightly, the safety was pushed in, impinged by the hammer and held by hammer tension. Cocking the hammer allowed the safety to spring out for firing.
My problem arose when the thumb of my shooting hand depressed the safety in my normal "thumb high" grip. My thumb depressed the safety enough for it to intervene between the frame and falling hammer, causing a misfire. Not the situation you'd want in an emergency back-up pistol.