Very interesting, Bruce. That is almost the exact same arrangement I came up with on my own. Of course, the main frame is left alone and my version is adding additional tubes(heavier!) to support the lateral arms. The way that is set up will give more negative camber on compression and extension, which is good. Setting it up at almost zero camber(but with a touch negative) should give GREAT handling. Add a 1/2" anti-sway bar(along with 5/8" or 3/4" front) and that thing should STICK.
In order for Beck to offer a non-swing axle rear suspension would require a complete frame re-design. They would have to forgo the torsion tube and ladder-type design completely. I doubt this will happen, unless suddenly front beam/rear torsion assembly supply dries up.
The market simply does not require it. Nor is the juice worth the squeeze for Carey and his company. The front beam/rear torsion/swing axle works well enough for them and their customers. I'm pretty sure he'll concur with me on this.
As to the history of the old IRS Vintage rear: The design came from ACI, which became Vintage when Greg acquired it. I'm not sure how many were made in either ACI or Vintage ownership, but I don't think many. The old design had equal length lateral links, which would not allow extra camber under compression and extension. I'll submit that this is why not many were built. I'll bet it had some handling quirks.