Cadcorp does indeed read over a 100 formats including SDE and Oracle Spatial directly on the fly. As Far as I know they read and write XML in the form of GML2, Gzip, LandXML which are all published Geospatial formats, they also allow you to store any Geometry in any database as OGC WKT, WKB and GML2 so this would enable you to store your data in a XML format in a RDBMS. Why do you want to export to XML, XML is not a specific format but a file type. What XML format or dataset are you looking to support ?
On the Polar coordinates, you can use Relative (X+5, Y+10, Z+5), Polar (Distance, Angle) or Cartesian (X, Y, Z) in Cadcorp for data capture through the desktop interface but I don't think you can access this through the Active X API, You could easily mimic this by simple doing your own maths. I.e. 100m at 45degrees from my curent X,Y,Z is not hard to work out.
hope this helps, give Cadcorp a Call, they are normally pretty helpful and send me code when I have asked similar questions in the past.