GeoParquet just embeds the geometry as WKB, so getting PostGIS geometry is as simple as going "SELECT geom::geometry FROM my_parq_fdw_table". The spec doesn't yet have anything to say about how to get a good spatial sort or carry out a spatial filter, so it's all pretty ad hoc on that end. Thanks to ogr2ogr, generating a GeoParquet file is now pretty straightforward.