Can you provide more details please?
All of the lifecycle based components within the ESB contain an initialize method which gets called on deployment of that .esb. If you're talking about something that will get fired even before that, then you could look at an MBean.
Specifically what I wanted to do is to initialize certain services before the ESB was accessible. For instance, to initialize a single shared Spring context so that Hibernate would have it's session factory activated, pre-populate a cache, etc.
As you pointed out, I could just have a single action class with an initialize method specifically for this purpose. I wrongfully assumed that the initialize methods were invoked just before the process method was, rather than when the ESB starts up.