A balance sheet is not something that needs to be forged. Every node should do it by itself - it has all available data.
The only thing they should do is send the hash of deterministically-generated balance sheet after block x. The version with most NXT behind it becomes the official one.
Ah yes - of course - thanks (should have realised this) so each and every (full) node will constantly be doing its own updating of a ledger and getting ready to "wipe out" the old history.
I guess the problem comes though in what do you send a brand new node who has no block chain (or do they need to download one from the last "checkpoint")?