When there is no transaction context, you cannot access resource manager.
After the transaction is 'commited' or 'rolled back' then only 'afterCompletion' method is called. since the transaction is already completed and there is no meaning full transaction (no transaction context) left for 'afterCompletion' method we could not call any resource manager.
I don't have the answer for the question you posted earlier. so i waited any one else in the group might answer but still i did not found the answer. So i posted the same in another forum. please find the conversation there.
I am not sure wheather it is good or bad to post the conversation held in some other forum at this place. But considering that the conversation will help you & others i am adding a link below to refer it.