I met Mr. Chapman at the "Builders Ball" show in the Boston Design Center. He was there with Richard Sachs and Peter Weigle- Chapman's work compared favorably with theirs, despite their more legendary status. I actually got into my career of gold/silversmithing because of my desire to build lugged bicycle frames. But the paradigm changed, while I was in school, to lugless welded designs.
This idea for securing a reaction arm is part of a series of frame issues that I have contemplated for more than thirty years. Many of the other ideas, I have seen implemented already. This one hasn't been addressed mostly because rear hubs using reaction arms weren't exactly commonly used by high end builders...
The shift towards more specific frame/ drivetrain options; IGH's, SS/FG, and Dual Drive (derailleur with IGH), and belt drive arrangements, has been a welcome development. A custom frame may actually be set up for something other than derailleur drive trains, by design.