… in older versions … there is no C exposure to the selection collection.
But, there always was exposure to the Ruby API. And Ruby itself is implemented in C.
So you can make SketchUp Ruby API calls from the C side.
You would need to find a way to convert the Ruby C types to SketchUp C API types.
(They made this easier in ver 2020.2 by adding conversion functions.)
See the downloadable Ruby Book list for PDF book:
“Extending Ruby 1.9: Writing Extensions in C”