rasterio.warp.reproject takes a kwargs parameter, which is passed to the "transformation function". It is not by any means clear what this means. What transformation function? What are the allowed values for kwargs? Can this please be expanded on? Thanks.

An option like $ fio cat --bounds w s e n --bounds-crs <crs> --dst-crs <crs> would allow for filtering prior to reprojection, like $ ogr2ogr -spat w s e n -spat_srs <crs>.

See Toblerity/Fiona#210 for a discussion about standardizing bounding boxes as --bounds w s e n rather than --bbox w,s,e,n.