@Faerballert 

Perhaps you clone your flowfile before the mergeContent processors. So whichever relationship you are connecting to your current mergeContent, you drag a second connection containing that same relationship to a parallel notification flow.

Down this parallel flow path you use a replaceText processor to replace the content with the value from the attribute you want to merge.  Then you use a mergeContent processor on this path to merge these files using a "," as your delimiter.   Then from this mergeContent you do you notification.

You may also want to open an Apache Jira with your use case and desired improvement for the existing mergeContent.  The more details the better.

Hope this helps,

Matt