Alright everyone. IT'S TIME!
Before you go hopping in and sieging everyone to your hearts content, ALL blocks are vulnerable to explosions. So essentially under the conditions needed to start a siege, once a siege starts your claim is completely vulnerable including chests. The defender still can't pick up drops or anything so make sure your always prepared to fight. The attacker can on the other hand pick up items and attack you in the claim.
Enjoy and happy raiding!