This is cool.
So for instance, if I was planning to use a bunch of different notes over the coming days/weeks/months, then rather than generate them on the website and back them up individually, I could generate a seed phrase locally and then just starting pulling individual private key/address pairs from that seed phrase to use as notes?
Yes you can do that.
I guess this is supercool feature.
You could even share an xpub with whirlwind to have your addresses automatically generated?
This is not possible but may I ask why would you want to do that? You can receive payments to any valid Note address even if you never accessed Whirlwind before, they don't have to be 'initialized' in any way. As long as you have the private key you can access the funds anytime.
Example: Imagine you need to receive a payment so you generate a new address locally and send the Legacy address to the sender expecting a normal Bitcoin transfer. The sender can now pay you instantly, anonymously and for free through Whirlwind even if you didn't know we existed. You could then access the website and withdraw your funds to your desired address. (the sender can also send you the LoG for the Pay to Note transfer proving that he sent the funds)
This feature will become even more useful once we decentralize the service and open-source everything. It may seem impossible to do this given Bitcoin's technical limitations but you know the quote: it always seems impossible until it's done.
We figured out a solution to make this possible and we are working on a research paper as we speak. We really hope we will get the chance to implement it, we strongly believe it would be a complete game-changer for Bitcoin privacy and perhaps the same model could also be used for other types of applications that involve custody of Bitcoin.
The only thing holding us back from starting the development is the modest usage we've seen until now for the current version. It already took about 4 months of intense development to get to where we are now and we are not yet convinced enough demand exists to make the effort of implementing the new version worth it, but hopefully this will change with the Anonymity Mining campaign and once people truly understand how Whirlwind works and why the model used is superior to any other.
If anyone is interested to continue this discussion please reply in this thread so it's easier to follow:
https://bitcointalk.org/index.php?topic=5444933.60