I have a well-protected masterseed that I have not memorized, and memorizing it won't help since it could already be compromised. My solution is to use a clever passphrase on top of that masterseed. I've been looking around in Mycelium because I read somewhere that you can appear to access your bitcoin wallet and show any bad guys that it's empty by using an incorrect passphrase. This makes so much sense that I want to do it, at least just for fun.

As I understand it, the masterseed encodes an integer which is the private key for a bitcoin address, and also a chunk of entropy that is used to generate the paths in the resulting HD wallet. When a passphrase is used, the effect is to change the masterseed so that an entirely different HD wallet is the result. So with one masterseed, you can have as many (invisible!) HD wallets as you want. The feature required is the ability to restore from the masterseed plus passphrase. Indeed, if the wallet is locked with the passphrase, then entering the wrong passphrase gives you a new wallet (and you'd have to realize on your own that you entered the wrong passphrase).

That is my dream: to be able to run Mycelium in front of a bad guy, enter my passphrase, and show that I have zero-balance. An additional feature would be the option for old transactions to show up so that it still looks real, but like everything has been spent.

I have an Android with Mycelium, and I've looked at the cold storage and backup features (and the "Advanced" add-a-key feature) and I can't find a place where I can enter my 12 masterseed words PLUS passphrase and get a new wallet. The only place I found like that is for cold-storage spending, and then it says "no funds found" (as expected). It could at least give me a btc address so I could send some funds to my special secret passphrase address.