AFAIK EJB asynchronous methods do not propagate transaction context (see 4.5.3 Transactions in spec). Frankly speaking I can not imagine how this (any context propagation) could work and what benefits it would bring.
As CDI 1.1 will surely have context lifecycle support there is a place for portable extensions to implement non-EJB async events (e.g. deltaspike module).
So I don't think it's necessary to define asychronous events in the spec. Nevertheless I do vote for independent execution with no context propagation.