I had thought custom fields for COA was the best (or at least most achievable) method of extending Managers reporting for application and jurisdiction specific classification. An actual use case is described in some detail here Custom fields for chart of accounts - #5 by Patch
However after thinking about it further I no longer believe that approach is optimal. While it could be used by the very technically skilled users, the majority of users will find such a tool set just too hard to even start using. The underlying issue is it does not allow separation of the component of the reporting common to a group of users / business (which should be shared as a localisation) from the component specific to a particular business.
A more powerful method which does not have this limitation would be to support
As described in more detail here Ratio Analysis - #12 by Patch