The more I look at it, the more it seems the concept of rulership at all doesn't fit into the modern schemata. Not a criticism. An observation.[/QUOTE]
Oddity, I think it's logically the reverse: Modern Rulership doesn't fit into the Traditional schemata. Your question seems to be: Does Modern Rulership HAVE a schemata of its own (or at least one you can personally relate to).
Leaving that question aside for now, Mercury in Pisces brings Mercury into contact with Piscean Rulership (whether it's Jupiter, Neptune, or both). If it's Neptune, and we associate one's "thought-process" with Merc., there would be a preference for the Big Picture, the forest first, THEN the trees, in that context. Jupiter is expansive and goal-oriented, and would bring greater clarity; still Big Picture, but less imaginative and more fact-oriented than Neptune. Together, not a bad combination! But, I see your point--the terms dignities and disabilities are Traditional, so Neptune, whatever its virtues as a Ruler of Pisces, doesn't fit this Forum, even though it's in keeping with the Thread from a Modern perspective. Maybe this Forum should be included in the Trad. category?