A few kernels involve the bounder and can be implemented fairly easily using the S2 RectBounder. - ST_AnalyzeAgg - ST_XMin/YMin/XMax/YMax - ST_Envelope (what to do about crossing the antimeridian? Probably a MULTIPOLYGON? - ST_Envelope_Agg
A few kernels involve the bounder and can be implemented fairly easily using the S2 RectBounder.