Problem merging tags

[MarmiteSandwich]

My control points only support a limited number of displayed tags, including Comment, so I am attempting to show Contentgroup, by merging (appending) Contentgroup onto Comment, so I can see the text from both tags in one display field. I know that Contentgroup exists, because I can use it as an indexTag. I am following the rules for the Tag values property, by specifying Comment.merge={Contentgroup} in the TagValue option field. If Contentgroup is not listed in ItemTags or IndexTags, I get this error message in the log:
incorrect tag name 'Contentgroup' in option 'Comment.merge' in property 'tagOptions'. If I include it in IndexTags or ItemTags, the rescan runs without errors, but the Comment field, as displayed on Now Playing, is unchanged.

Am I missing something, or barking completely up the wrong tree?
Marmite