I'm not sure if there is a good way to do this FR studio, but an alternate suggestion is to create your report with the first two letters in FR studio; then open your FR studio report with all the formatting into Excel (with the "refresh using workspace..." option checked if you want it to dynamically refresh), then on a second sheet of the Excel workbook link your cells to the FR studio report. Then use the excel format painter to bring in the formatting from your FR Studio report. The format painter messes up the column widths, but depending on how big your report is, this can be fixed rather quickly.
Of course, if the first two letters are always the same, just import the file without checking the "refresh" option and just do a mass replace of the letters.
The other alternative is to put the numeric part into a user defined field of the metadata item or as the actual description of the item and pull those into your custom header. In my opinion, both of these seem like a frivolous use of a powerful system .
Hope this helps.