You don't want to access external block chains from inside your altcoin client, that would be an unnecessary security burden.
That is my suggestion. But only in some ´distribution´campaign period, like the 3 years of Freicoin initial distribution. A further sophistication would be to arrange this as a Bitcoin 2.0 bond auction: You auction off a certain amount of Bitcoin 2.0 per month, generated by Bitcoin 1.0 pledged and ultimately transferred by destruction tx´s in that period.
You can scan unspent output up to a certain height in bitcoin, and aggregate them into a large genesis block. Trim the noises to make the dataset size reasonable (say only outputs >1btc).
Would not that benefit exactly the early 1.0 adopters, if you set the height low? Or the hardcore hoarders that did not ever sell a single coin?