"Ma'am coming from the word Madame, the wife (or "Dame") of a lord. The French follows suit: monsieur et madame.

I follow, unfortunately, modern rules of decorum and will address most everyone as sir or madam. But a common term for both cannot be by nature of the rules of those Feudal times. Women never had official power, only by proxy."

No. Acknowledging official power as private property, and as such women as potentially (if rarely) the holders of official power was actually a feudal innovation.
A Dame might be a widow of a lord. Or a daughter and heiress of a lord.
And it might be a man who holds official power by proxy, de jure uxoris.
Another title is "mademoiselle". Meaning "a little madam".

Do you like to address a girl as Mademoiselle? She should hold power. Especially hold power over a man entrusted to fondle her fun button.