How can I access the BBS instance that wraps my business object at runtime ?

If you will use the new Custom Business Binding Source the internal BO is exposed through the SourceBO property.  You must be on the most recent 1.6.6 beta build, but this will handle a vast majority of the logic that I think that you are trying to code manually at the moment.  There is a new template for this as well when you add a new item (Custom Business Binding Source).  Then in code you can access it like this:

Talking to the strong-typed properties

Talking to the standard BO methods