How to filter work item types when using WorkItemUI.createWorkItem() ?

Jazzers,

This is about RTC client-side extensions in Eclipse.
In the context of a process I am popping up a UI which allows the option of creating a new work item. This UI makes use of WorkItemUI.createWorkItem() as detailed in the Programmatic Work Item Creation wiki.
Since I want to prefill a custom attribute in the selected work item I would like to build in a filter routine to only display the work item types that support this custom attribute. I looked at the code but the NewWorkItemWizard being used there is already deep within internal API.
It seems rather non-trivial to extend this with a filter function.

Grateful for any general ideas on how to implement this filter short of writing my own wizard.

thanks,
Arne