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")?
Unfortunately, the only answer is: they have to download the blockchain from somewhere, starting from last checkpoint (or genesis) in code. They just don't have to store it.
This means that full blockchain still needs to exist, just not on all nodes. Perhaps nodes with full blockchain can get a boost to their forging chance, as a fee paid for blockchain storage service? Like 2x their normal chance.