These are chat archives for PDAL/PDAL
As I understand it though, @mrosen has a second file containing only Classification, with no coordinates. But as you can see in https://pdal.io/development/overview.html#dimension
In order to be useful, all formats must provide some notion of location for points (X, Y and perhaps Z)…
Even if you had coordinates in the second file,
filters.ferry would not work, as you cannot ferry from one PointView to another.