I have looked into this, and I can't find anything on it.
What the Fireworks developers could do to help extension developers is to provide access to several arrays in the Fireworks document, such as the collection of Library items, layers, and pages.
One for the developers, I think.
Yup, I've asked for an API to access the selection in those panels for years, to no avail.
Note that you can loop through layers, frames and pages, but you can't tell what the user has selected in the actual panel UI. Also, you can't find out which symbols are currently used in the document. Nor can you programmatically insert a rich symbol into the document and have it maintain its "richness".