Hi Gunther,
well, let's try: Additional accounts for all US GAAP deviations - along with balancing accounts for these deviations, so that these will be a closed group of accounts where US GAAP deviations are posted to. From what I understand from your setup, this would eliminate the "consolidation" step to see one company in US GAAP (you can do this with account schedules). In effect, you would have two COAs (and differential books) living side by side, not touching each other - but in one company.
Regarding Air Berlin: No, it was a custom job. I toyed with the idea to build a commercial product out of it, but it didn't work out yet. Too small a market for the vast effort. I know how to do it, though :)
Regarding Force Dimension Balance: It is deep in the core of the posting logic. You would need to do a few things:
1. Make sure that the "BOOK" dimension is always filled in (as you write). This can have some issues with system-created entries, the posting routines need to be checked that they use the correct dimensions for VAT, discount, adjustment postings. Also, Close Income Statement (for example) must be posted by book dimension. And a few other details. NAV2015 in the higher CUs is pretty good at doing it right. Not perfect, but good.
2. Make sure that the balance check is by date, document no. and book dimension value. If not, a consistency error must be thrown.
3. Make sure that you have "clean" subledgers (customers, vendors, fixed assets, bank). Transactions where these take part can only have the "local" book dimension. This avoids a lot of nasty special cases where the posting logic would have to be upped a few dimensions in complexity.
All localized NAV codebases would have to be checked and tested against this, off course. From my experience, once you have good W1 implementations of your additional ledgers, this should be not too much of an effort, provided you're using a modern source control system.
All in all, I would try to build it with the separate accounts method. GAAP deviations are only a few accounts compared to a complete COA, and ensuring that you can't mix these accounts with your normal posting stuff is easy. Consolidation accounts are simply the same as the normal accounts. It is the smallest possible customization while gaining what you intend. The other methods are more powerful (parallel ledgers is vastly more powerful), but it adds complexity while doing basically the same thing.
If you need more information, please contact me by private message.
with best regards
Jens