Let me clarify my reasons for wanting this setup. I occasional do panoramic photos, but this isn't the primary reason I want a pan base on top of my tripod head (older Arca B2). I do architectural interiors, and setting the camera orientation normal to certain architectural features is important to me. By far, the easiest way to do this is to find some horizontal feature (ceiling or floor junction, counter-top, windows, etc) and rotate the camera until a horizontal grid line on my ground-glass lines up with the feature. Unless you level the tripod, put a leveling base between the head and the tripod, or use a panning base on top of the head you end up going off-level as rotate the camera (like chasing your own tail). Having read previous posts, it sounded like a panning base on top was a good choice.

I also spoke w/ Arca-Swiss about their concept. I guess the product is still in flux. In any case, they didn't give me a high degree of hope that this future potential product will work with older style B2 heads (over about a year old). I suggested to Arca that one of the things their customers enjoy is the backward compatibility they have with many new components, and not supporting older B2's was inconsistent with their approach.