Was unable to find detailed documentation on the subject...
Does anyone know if use of the SingletonStatelessSessionInstancePool implies singleton behavior of beans using that pool only within a single server node or across entire clusters?
Where have you found this class: "SingletonStatelessSessionInstancePool"
Anyway, I don't think it is clustered (I didn't do it myself, maybe someone else did it). Anyway, if you want such a think, it could be easily implemented through the DistributedState service.