You can do that with xPriv paper wallet, but unfortunately there's no BIP38 for that. There should be.
So I scan the xpriv QR code into Mycelium, it checks which unspent outputs exist for that xpriv, picks some, and sends change back to another of the addresses generated from the xpriv code?
That sounds like a good solution, except that due to the lack of encryption in the xpriv QR code I have now gone from losing all my coins if my phone gets stolen to losing all my coins if the QR code gets stolen.
I was hoping for an HD solution where at least some of the information needed to spend the coins is in my head. Like as a passphrase.
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You can already do something very similar, not with xpriv but with entire seeds.
Just generate a BIP39 word list and put that into a qr code (hint: mycelium entropy can do this).
Create one or more passwords however you prefer to.
Scan the seed words QR code, enter optional password, select account, spend.
That way, you have a HD wallet on paper, password required, each password gives another wallet.