Dear Paul
I believe using "condition" techniques in combination with your customer report symbol: you can do the job (e.g. based on my proposal; i have done a short check and it does work (not using customer specific symbol clealry) and it was quite easy to prepare WWi layout)
Once again I would like to give you hints in the "cons" of this approach (and there is from my point of view no "pro"):
a.) using WWI techniques you can not search for the data shown in the report in CG02 =>> clear con
b.) during any generation of a WWI report the calculation is done once again. E.g. imagine that you prepare today a report in "FR" and tomorrow in "EN" and in the meantime somebody has added/changed data (so that now average value is maintained): you will not get this "explained" in short time (that means contentof WWi report one in comparison to two). => clear con
c.) depending on the vaculation rules you are using you might have a "lot"of trouble. How to "intepret" an "empty" value? Imagine that no average is maintained nor lower value but upper value only.What is the result of calculating the average in your approach? => clear con
So from my perspecitve. there is no "pro" (ok the user might feel to be supported but if you really discuss with him/her than he/she should be "unhappy" soon with this solution.
So either by using "maintenance" rules or by using the BADIs which I have mentioned you should solve your problem in maintenance process. If you use e.g. expert rulesets in any case this is the right option to look for.
C.B.