I've been thinking on this for a long time and haven't found a way so far. It's easy to "peg-in" to a BTC address from Bitcoin. The The crux of the matter is that there is no way for the Bitcoin script to validate the release of a pegged coin when the new owner of that coin, e.g. an ecash note, wants to peg-out and back to the Bitcoin main. (I use the established Liquid terms for peg-in and peg-out. Personally, I would reverse them as I see the transaction fundamentally as Out of Bitcoin and then back In to Bitcoin. The current terms seem layer 2 centric, but I prefer BTC centrism.)