The Features menu is only for previewing GSUB features. The kern feature, like all other GSUB features, is generated at export. This is also when the custom kern is inserted as a separate lookup into the kern feature. So there currently is no other way to preview it.
Set up a sample document in InDesign, set the Adobe Fonts Folder as export destination, and all you need to do is press cmd-E, and see the font update in InDesign.