Unfortunately, it isn't possible to use session-scoped beans in one of your normal bundles. The use of session (and request) scoped beans requires a Web-aware application context which your normal bundles will not have. There's some
more information about this in Spring's reference documentation.
Your solution of using a custom-scope for this sounds like the best solution at the moment.
We're very interested in modular Web applications, and your requirement is an interesting one. It'd be great if you could open a story in our JIRA that describes your use case.