The bbCode editor was a bit experimental for V4.0 and I can understand the desire for some people to extend it.
Doing so is fairly straightforward and involves 4 files. The actual editor javascript file contains the code that handles the actual editor buttons and process. There is also an 'init' file for any text strings to ensure they can be localised. Then there are two parsers – one to convert the bbCode to HTML and one to convert HTML to bbCode (for editing of existing posts).
It is very unlikely that we will be 'officially' adding any new features to the editor in 4.1 but if any user wants to code up a new button and the necessary parser entries please do send us the code and we can consider adding it to the official release.
It is worth noting that the parser is being updated in 4.0.3 coming very soon. This change is not in the current beta 2 of 4.0.3.