FlexCelReport should provide a new method "GetValue(string tagName)", which allows to read a cell value from the underlying Excel file. In the Excel file there should be a special named worksheet (e.g. <#ReadTags>) whit four columns "TagName", "Worksheet", "Row", "Column". On this worksheet, the user could define the mapping between the "tagName" (of the GetValue()-method and the cell where FlexCelReport should read upon request.
I am not sure if expressions and <#= > wouldn't work here?
You can define an expression in the config sheet (columns M and N):
tagname: <#=(MySheet!D11)>
And then in the template use <#tagname>, it will refer to MySheet!D11.
Note that as always <#= ...> will work fine for cells that already exist in the template. For cells that FlexCel will fill when running the report it is problematic, because you don't know if the cell has already been filled by FlexCel when you refer to it. For cells filled by FlexCel, you should use directly the <#db.fieldvalue> tags used to fill the cells in the first place.
As it is now, I don't see a big advantage of having a <#readtags> sheet over just defining them as expressions in the config sheet. And if possible, I would prefer to not add features that don't contribute much to what is possible.
So please let me know if using expressions would be fine in this case, or if there is something else I am missing, which is quite probable.