The moment I transfer a bitcoin to Silk Road, it's no longer my bitcoin. It's theirs. I've exchanged it for a promise from Silk Road to pass on the bitcoin to someone else or return it to me when I ask.

The government never seized my bitcoin. They seized Silk Road's bitcoin. This means Silk Road has more liabilities than assets. And since Silk Road hasn't given me a promissory note in exchange for my bitcoin, I'm an unsecured general creditor - and unsecured general creditors cannot establish standing to recover seized assets.