Earlier this week, I helped a customer who wanted to use PeopleSoft nVision to do ledger-based reporting in EPM. Because their ledger and business unit structure was the same between their PeopleSoft Financials application and EPM, this was pretty easy.
The secret is the view NVS_LEDGER_VW, which nVision uses to find all the info it needs to access a ledger. The customer updated their Ascential maps to load the tables that were used in this view, and nVision magically worked.
There are some caveats to this approach, however.
Another approach is to do Query-based nVision reports (which is how the delivered EPM nVision reports access data). However, Query-based nVision reports do not know how to use timespans (which is pretty important).