Doing a little homework, thanks to bazar01 on this forum, I found this picture and the phasing weights and Hall sensor come out as a complete unit after removing 3 screws.
The builders of twin plug distributors basically remove the corresponding set of parts from a stock SC distributor to build twin plug units for early cars.
It seems it could be done for the 3.6 just as easily, maybe with minimal modification to existing conversion parts.
Also, not to put too fine a point to it, but at least for the '89 C4, the factory "Information Technik" states about the "Double Ignition" on page 2-15:
"A hall sender, which is required for identification of cylinder no. 1 (ignition TDC) is installed in the double-ignition distributor. The hall signal is required in the DME control unit for assignment of both knock sensor signals
as well as the control of sequential fuel injection.(my italics)
Each distributor rotor is guided by centrifugal weights depending on engine speed. This guarantees alignment between the distributor rotor and corresponding ignition contact in the distributor cover."
This seems a good product for someone to develop...Steve???