The Developer Kernel supports transparency on per-layer basis. It is possible, however, though not very efficient, to force a transparency for a single shape.
procedure TForm1.PaintShape( const _sender : TObject; const _shape : TGIS_Shape );
var
h : Integer ;
begin
h := GIS.LockTransparent( Random( 100 ) ) ;
_shape.Draw ;
GIS.UnlockTransparent( h ) ;
end ;