Taylor,
When you consider the course of Alembic's history, based on design, plus when you crunch the numbers their isn't much room for that elusive 3rd pickup.... Back in the day, even if the dummy coil was placed under the tailpiece, it would be near impossible to fit 3 AXY shaped pickups on a short scale bass. Moving up to 32 scale length makes 3 pickups an option, but aside from Phil Lesh's Osage Orange, it wasn't a feasible option back in the day, unless you ditched the PF-6 board and went for an entirely custom electronics package.
Once Alembic started to branch out into their current model lines, having a 3rd pickup became a viable option (longer scale lengths, activatior electronics, various pickup molds, etc etc....)