Could a hybrid electric steam plane be more efficient at altitude than at sea-level, and how to optimise it to frigid air?
Forget the electric stage - when using steam, you will most likely already have mechanical energy and when ducting that steam through a turbine, even rotational energy to turn your propellers. What advantage do you expect from converting that rotational to electrical and directly again to rotational energy?

That design could be simplified even more: Instead of heating water, the fuel could heat compressed air and let that run through a turbine. That is how any modern jet engine works.

In order to keep mass down, the steam would need to be condensed in a cooling circuit. While the wing surface could be used for this, the required piping would add a substantial weight penalty. In an open circuit where the relaxed steam is simply dumped overboard (like on the Besler biplane or most steam engines), range would be rather limited.

There was to my knowledge a single attempt at building a steam-powered airplane with steam condensation. This was a version of the Messerschmitt 264 bomber which was supposed to run on a mixture of coal powder and heavy oil instead of gasoline. It was never completed.