This issue persists for the BIFF (XLS) file format. This issue has been resolved for charts in the OOXML file format (XLSX, XLSM) as part of implementing OOXML support for ExcelApplication in OfficeWriter v8.0.
From that point on you can programmatically change the label as you like using the properties. However, if you open, with ExcelApplication, an existing workbook which already has a Trendline label these properties will work properly. Setting these properties to true does not seem to have any effect and the label is not shown. The Trendline object has the properties to ShowEquation and ShowRSquaredValue. Trendlines can also be created as objects in ExcelApplication through the Series of a Chart. It is possible, in Excel, to add a label to a trendline of a chart that shows the Equation or the R-squared value of the trendline like in the image below:
