Dear Emanuel,
Thank you for your query.
Sorry this went unanswered for so long.
Afaik, you can easily access the original geometry during the custom export process.
Each graphical object exported is equipped with property data pointing back to the originating element.
You can query that element for its geometry in the normal manner, and associate it with the exported facts or whatever.
The custom exporter gives you all the basic geometric information that is needed to render the scene in a manner similar to what Revit's.
If you need additional information or access to the original geometry, as you requested, you can use the Element.Geometry property (which generates the get_Geometry method) and other standard access methods just like you suggest.
Be aware that when you query the element directly, the coordinates you receive may be local, not global. The local coordinates normally need to be transformed to global before you can use them. Therefore, the ones provided by the custom exporter are normally easier to use.
For the sake of efficiency, I would implement a cache containing all the original element geometry that you retrieve, so you can reuse it immediately if the same element causes several different facet exports.
I hope this helps.
Best regards,
Jeremy