We prepare total rewards statements for client and have been doing this for several years using Word/Excel, so we know it works. We haven't yet figured out how to do it for online yet due to a step in the current process that is inherently manual (updating the fields) but I stumbled across your post while looking for ways around it. Here's what we do:

Store the charts with a name that includes an identifier field. We use empIDChartname, for example 12345Pay

In the Word doc, your field is {LINK Excel.Sheet.8 "\\\\drivenamen\\directorylevel1\\level2\\levelN||{MERGEFIELD "EEID"}Pay \p \* MERGEFORMAT}

Then you merge your doc, and update the fields (Ctrl-A, F9) to get the charts to pull in.