Can you add value to a Casascius physical coin? The firstbits address on the front of the coin (once fully resolved from blockexplorer) is just like any other address right? I can buy a 1 BTC Casascius physical coin, copy the address, bury that coin in my backyard, and use regular transactions to keep increasing its value?
Yes, you can add value.  The block chain only knows public keys. It doesn't care how or where the corresponding private keys are stored.