I'm going to run with this idea for a moment.
Lets call it Block Chain Link Fencing.
The ways to implement might include things like
1) merge mining the new chain
2) Finding the the unspent transactions
3) providing a way to move unspent transactions from the old chain to the new chain
There are other solutions already in play such as the use of bloom filters to prune the old chain in the software. But I thought the name that popped into my head for this feature was clever enough that I should write something for it.
(we are either fencing coins, or fencing off bloat by linking block chains, take your pick)
But you seem to be only concerned with storage size, which I pointed out appears to me to be the uninteresting part of the problem. As far as I can see so far in my investigations and calculations, the bandwidth cost appears to be the greatest hindrance if you are talking decentralization.
And when you dig down into how decentralization plays with everything else...
Damn, they are forbidding me from posting again. Bye.