Unfortunatelly these custom fields not readable by excel, only with fps, but I think might be useful these hidden properties.
In Excel 2016 they are readable, and I think that they are also in your Excel 2013 because similar items in the xml files exist already in the pre-xlsx XML format of 2003.
The problem is that Microsoft has an awful user interface here. In the metadata screen of my version there is a button "Properties" with a dropdown menu. After clicking on the only item of that dropdown a dialog appears in which the custom properties can be viewed and modified. See my screenshots of the file created by the meta data demo program of fpspreadsheet (they are in German, but I hope you can locate the controls nevertheless). Note that fpspreadsheet stores custom properties only as strings while Excel (as well as LO Calc) allows to select a data type for each. But I think this would make things too much complicated for such a rarely used feature.
I noticed one small bug, Keywords separated with "," excel: "First,Second,Third" but the comments merged without any separator: excel: "FirstSecondThird'.
excel corexml: <dc:description>First_x000d_Second_x000d_Third</dc:description>
The "_x000d" is the replacement of a line feed used by Excel here. Again, when you look at the comments in the "hidden" dialog mentioned above you will see the line feeds. It is hard to understand that a company like Microsoft does not care for a better user inferface here considering the age of "Excel". Are we two the only persons seeing this?
BTW, you are talking of "keywords" here but your input shows up in the "description" field. "True" keywords are entered in the Excel interface in the box "keywords" as a comma-separated list. My field "description" (named after the LO Calc field) gets what is input in the "comments" box.
Another BTW: I see in the "excel_metadata.png" screenshot that there is an option to add another author ("Autor hinzufügen"). FPSpreadsheet ATM supports only a single author. Do you think it is necessary provide this option, too?