You (probably) may receive tag values you want via exif callback: https://www.libraw.org/docs/API-datastruct-eng.html#libraw_callbacks_t and https://www.libraw.org/docs/API-CXX.html#exif

As mentioned on the this site homepage/LibRaw project description:

Additionally, the LibRaw library offers some basic RAW conversion, intended for cases when such conversion is not the main function of the LibRaw-using application (for example: a viewer for 500+ graphic file formats, including RAW). These methods are inherited from the Dave Coffin’s dcraw.c utility (see below the “Project history” section); their further development is not currently planned

LibRaw RAW postprocessing (sample code) do not apply any custom aspect ratio change, although you may implement your own on processed image.