Yes, the Priestess would be the alchemical mercury, which is female. The alchemical mercury is - in addition - not the metal we know, but a materia prima, a lunar metal often represented (in alchemical texts and plates) as a serpent, a half-woman half-serpent (like the sorceress Mélusine) or a mermaid (fools_fool - have you also read what Paraceslus wrote about that aspect of Azoth?). Hence - the link to the Priestess, a lunar card; it is at the basis of all transformation of other elements - and of the alchemical conjuction (marriage).
Frank, you are asking a wonderful question, and one that has been working in me in the past few days, though in my case through the study of the Lovers, that rich and complex card where so many myths and ideas play together and against each other. The marriage, of course, might also be seen as that of salt and sulfur - The Empress and The Emperor - but the materia prima, the alchemical mercury of the Priestess is implied in the card too, just as the Alchemist-Hermit is expressly shown. The serpent around the Orphic egg in the Lovers card echoes the serpent of alchemical mercury and the serpent-lilith that seduced Eve (both Lilith and Eve are in the top corners of the Lovers card; the serpent is at the bottom, curled around the Orphic egg).
When I look at the Priestess, Empress & Emperor, I get a strong sensation of order and balance, a world that works like clockwork, in ordered movement (thank G-d, then, for the mad Fool and the creative streak of the Juggler!), which is strongly shown in the Lovers card (the latter, according to Crowley, represents the Creation of the World)
BTW, II-The Priestess, III-The Empress and IV-The Emperor together come to IX-The Hermit, the alchemist who officiates at the marriage of the Black King and the White Queen in The Lovers.