Mark, I am not particularly interested in the history of the tarot except insofar as it influenced the history of astrology.
I am fascinated by Phillip's interest in the earliest mention of the Pisces-Neptune connection. But historically I think it is equally important to consider when this connection took hold in the wider astrological community, which seems to have occurred later. I wish I had more late 19th/early 20th century astrology texts to consult, but I think Alan Leo (1903) is really important because he was the great, early popularizer of the new western astrology (for better or worse.) Leo's interests in the occult and spiritualism appear throughout his book.
I just took a second look at him.
I haven't found that Leo was a member of the Golden Dawn, but he was at least heavily involved in similar organizations and knew some of its members.
http://www.skyscript.co.uk/Alan_Leo.html
I think that when you find several parallels (re: planet, sign, tarot) co-existing around the same time and pointing in the same direction, you can assume either that this is mere coincidence, or else that there were mutual influences and cross-currents. I think the latter is more likely, given the small community of occultists in Britain at the time; and that therefore the tarot question has some bearing on Neptune as the modern ruler of Pisces.
In taking another look at Leo (1903) I found he was within a hair's breadth of making the sign-rulership connection. Even though he denies that Neptune rules any sign (p. 37,) his descriptions of Neptune (p. 37) and Pisces (p. 20) are close to the point of using some of the same words. Then in his discussion of the 12th house (ch. 19,) we find:
"This is not altogether a bad position for Neptune, being related to the sign Pisces, with which this planet has much sympathy."
This is pretty darn close, especially given modern astrology's weaker affection for actually working with sign rulers (for example, in dispositor and house cusps) and increasing tendency to conflate signs and houses.
Then if we look at the tarot, one of the early expounders and a Golden Dawn member, Arthur E. Waite, spent a great deal of his
Pictorial Key (1911, but based on much earlier work,) disavowing earlier publications purporting to explain the cards' esoteric significance. But he himself held those tarot cards and their meaning very close to his chest. If you read his description of The Hanged Man card, however, especially in light of the Golden Dawn link I posted yesterday, we can trace further parallels with the Neptune and Pisces descriptions in Alan Leo.
Waite points out "the tree of sacrifice," "life in suspension... but not death, "higher nature," "great awakening," and "the mystery of resurrection." All of which sounds sounds rather Christ-like.
To Leo, the succedent houses were part of a "psychic trinity" related to "intellectual conceptions apart from worldly knowledge or practice," and the 12th, in specifically related to misfortunes such as prisoners and self-undoing (cf. the head-down position of The Hanged Man himself was initially used for criminals), also relates to "mental sensitiveness to unseen worlds is awakened." (pp. 226-7.) Misfortune is turned to universal love, which not lonely echoes Pisces, but the Christ-figure of The Hanged Man.
Anyway, I am not here to stretch a point. Any actual connections would need to be demonstrated. But I find these parallels highly suggestive; and think that in the theosophical movement, if not precisely in the Golden Dawn itself, the Neptune rulership of Pisces had its origin-- but not necessarily its acceptance in the early 20th century astrology community.
