How do I stop Unity Dash truncating file names?

The answer appears to be No. Dash determines if you are using a desktop, laptop, or netbook, but after that the layout of the Dash results view is hardcoded. I saw no options available to change the layout or how it works.

Given the need to present a consistent appearance across platforms, especially phone and tablet, I can see how this is needed. I suspect such long file names would be a problem even for the equivalent of Nautilus compact view.

But you might file a bug report asking for the ability to see the full name in a tool tip or in the new preview view.

Another option would be to wrap a basic GUI, even something as simple as Zenity or Yad, or other GUI options using Python, Tcl/Tk, etc to create a simple way to browse and select these files.