[CONNECT C++] ECSchema GCS Coordinate Property

I want to annotate a point (e.g. a cell) with its coordinates in an associated text label. I can see in BaseElementSchema that an ECClass named MstnCellProperties has a property Origin. The origin is a point3d.  Here's an image of what I mean: the blue point cell O is annotated with its XY coordinates...

Annotated Points

If the DGN model has a Geographic Coordinate System (GCS) assigned, what property should I seek to find the longitude/latitude or easting/northing?  That is, in the above image, what should I do to display the coordinates of the green point cell G with its easting and northing in that GCS?

Formatter Classes

In the MstnPropertyFormatter there's a Point3dClass formatter class.  It offers ways to format a point3d's XYZ coordinates.  But I don't see a GCS formatter.  In fact, the only usage of GCS in any ECSchema concerns raster referenences.  What have I missed?