Some airplanes have radar that can shows the horizon. In a pinch, you
could keep the plane level with radar. Pitch can be maintained with
airspeed or groundspeed, with some caveats. In level flight, constant
heading, the wind doesn't change much. Monitor your GS, get iced pitot
tube and loose airspeed indicator, you just maintain your same GS as
you had before the ice, and you are good to go.