BTW - said "genesis" block is going to become "huge" in the future (if we had one million accounts then surely we are talking 200-300 MB which you are not very likely to be able to be sending to nodes very quickly - or are we working on some sort of special format to "compress" a genesis block?).
Yes, but there's no need for that.
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.