Barleywine
Others have suggested Mary, although not specifically the Shekinah - more of a Grail mystery. Florence Farr wrote a GD teaching paper describing an astral journey to Mary and an experience with the Grail containing the blood of her son. It's in my book Women of the Golden Dawm. I've guided large groups through the journey. I found some interresting stuff that may be more or less related in Eckhartshausen's Cloud on the Sanctuary - a book that Waite refers to often and which contains some interesting stuff on the Order of Melchizedek.
BTW, the guy who came up with the M-Theory in physics has never revealed what M stands for but suggests it might have multiple referents including Magic and Mystery. Coincidence? What if there are no coincidences.
In Volume II, Section IV, Part II of The Practical Guide to Qabalistic Symbolism Gareth Knight talks about the Grail symbolism and relates the entire suit of Cups to the Celtic sea god Manannan. He doesn't mention the Shekinah as such, but does use the term "the Feminine-Side of God." He makes no connection between Manannan and the "upside-down M" on Waite's Ace of Cups, though.