The documentation on the Create Constrained Issue seems to suggest that the getBehaviourContextId method should be defined for all behaviours. While the method is valid, it will always return null if the web item that launched the create issue form isn't a Create Constrained Issue web item.
It would be useful for all custom web items to provide a context ID, so that other web items could leverage this same functionality.
Likewise, for those contexts where it is applicable, it might be useful to provide a context issue ID for getContextIssueId() when there is an issue in the current UI context. We'll need to investigate whether that is feasible, though. If it's not, we'll at least need to document that it only works in the context of this web item.