Our first attempt at digitizing bent pipe structures that we had created with the HoloLens was during a workshop at Tsinghua University for CAADRIA in 2018. The small pavilion we designed for the conference exhibition was to be fabricated in parts, and to ensure that we would have a neat joint between these chunks (as well as to evaluate the precision of the fabrication process) we needed a method for creating digital models from as built conditions. We achieved this by 3D printing a mount for a tracking cube that would snap to the 16mm diameter of the bent pipe. Because we knew the exact geometry of the mount, we were able to offset the tracked position of the tube to find the exact center point of the physical pipe. To digitize the structure we then took sample points along straight segments of each bent part, and filleted these straight sections with the same radius as our bending die to create an exact digital copy of the physical object. It is an extremely low tech but also pretty effective way to produce digital models to within a few mm of the actual physical object. We were able to verify this by overlaying the physical structure with the digitized model - you can see a very short clip of this in a video of the project:
This approach generalizes to digitizing other structures made from stock material, so long as the precision required is not greater than the margins of error of the tracked marker.
We published a paper on the project that outlines the approach in detail. You can read the full paper on research gate: https://www.researchgate.net/publication/330158065_Making_in_Mixed_Reality