That’s a very ambitious request. I cannot imagine how Custom Fields could be used within the context of an already detailed and comprehensive report such as the cash flows or the P&L.
I can only assume that you will get quite a handful to look at.
Maybe if you provided an example of what you intend to have we could better understand your use case.
Also, there’s been some recent changes to reporting so maybe you could give Advanced Queries and the new Transactions view a try and see if it could give you the report you’re after:
Edit: I have to mention that Custom Fields support in the Transactions view is yet to be confirmed, however, you can use that view to have greater control over the report layout using Advanced Queries