'Liege' seems to most often refer to a man. Is that because most lords in history and fiction are men, or because 'liege' is a masculine noun? If the latter, is there a feminine counterpart?
To my knowledge, there is no feminine form of the noun liege. There are obvious historical reasons for this, but I lack the knowledge to give an apt explanation.
However, the decidedly masculine phrase liege lord was also quite common and more or less identical in meaning. Although I have never knowingly come across it in reading, the obvious feminine form of this is liege lady. Google Ngram leads me to believe that liege lady is used at least occasionally.
The more direct feminine form would seem to be liegess, but I have never seen or heard it used, nor does Google Ngram display a single instance of it.
Liege (adj.) word used by a vassal to address his superior or lord in the feudal system, c.1300, from Anglo-French lige (late 13c.), Old French lige "(feudal) liege, free, giving or receiving fidelity," perhaps from Late Latin laeticus "cultivated by serfs," from laetus "serf," which probably is from Proto-Germanic *lethiga- "freed" (cognates: Old English læt "half-freedman, serf;" Old High German laz, Old Frisian lethar "freedman"), from PIE root *le- "let go, slacken" (see let (v.)). Or the Middle English word may be directly from Old High German leidig "free." As a noun from late 14c., both as "vassal" and "lord." Hence, liege-man "a vassal sworn to the service and support of a lord, who in turn is obliged to protect him" (mid-14c.).
It's origin, anyway, seem to support your view on its use being mainly referred to men.