I can see the value in avoiding exposing the API unless necessary. It'd be an awkward and nontrivial bit of extra work to make this internal, though, given how the descriptor info is currently accessed by vkd3d_shader_parser_compile().
Maybe not trivial, but it doesn't seem that hard? In particular, I imagine we can essentially just pass the new structure as optional parameter to scan_with_parser() and use it instead of "local_descriptor_info1" if present.