Local transaction creation has to be allowed during graceful shutdown. e.g. if a web request is in the process of running and it attempts to start a transaction this must be allowed (the core requirement of graceful shutdown is that requests that have already been accepted continue to run as normal).
The only case when transactions should be disallowed are remote transactions, such as remote EJB and CORBA, which I think should already be dealt with at the respective endpoints (in terms of disallowing new transaction creation). I think the main thing that needs consideration here is what to do with EJB requests that would otherwise be rejected that are part of an existing remote transaction. We probably need some way of identifying these requests and allowing them to proceed.