Hmm, hoping that Siddle can come on and bowl at least a few overs even if he has to sit the morning session to rest up. Let's see.
As much as I would like to say this is possible, I'm fairly sure this is against the laws. The other thing about that is, if he was to sit out the morning session and be able to come on after lunch, he has to wait a certain period of time before he can bowl again. I think it's the overs, so assuming Aussies were to bowl 25 overs(I could be wrong and it could be time, but it works both ways in the same way) in the morning session. He then has to stand on the field of play and be active for that same amount of overs before he can continue to bowl. Overall, it's a hard logistical situation to over come, but it might not be worth it in the long run to try it.