I want to be able to unlock my private keys for a specified amount of time by typing the passphrase. The same as Bitcoin Core does. I spend from Mycelium maybe once a month. I can handle typing my passphrase that often. I type it into Bitcoin Core more often than that, and it's not a problem at all.
Use BIP38 encrypted paper wallet, which you can spend from using cold storage option in Mycelium. Change goes back to the paper wallet address.
Address reuse? But I want an HD wallet, not an old-style address-reuse wallet.
You can use bip 38 to create an HD wallet - just use the decrypted 64 char hex private key as entropy to create the 24 word passphrase
Could you outline the process please? I use an offline machine without network access to store encrypted private keys for cold storage, and Mycelium for online storage. Mycelium is unable or unwilling to encrypt its private keys, so what is the system you are proposing as a convenient alternative?
Using bitaddress.org (
https://github.com/pointbiz/bitaddress.org):
Create a bip38 encrypted single private key using the 'Paper Wallet' tab and save it somewhere.
Copy the encrypted private key and open the 'wallet details' tab and paste; enter the bip38 password to decrypt.
Scroll down and copy your Private Key Hexadecimal Format (64 characters [0-9A-F]).
Use
https://github.com/bip32JP/bip32JP.github.io to create HD wallet:
Paste the Hex private key you just copied as the entropy.
select the 'Base 16' radio button and '24 words' below Bip39 passphrase.
Click 'Generate New Phrase' and a 24 word phrase should be generated.
Double check you can repeat the process with the same result before commiting any funds etc.