It can be implemented, even if it is not so easy to implement, because it also effects the saving of the perspectives.
I have a working patch for this, even if I do not use this feature.
But we will use wxAuiNotebook instead of wxFlatNotebook in the near (?) future.
I had to patch the flatnotebook sources to get the double-click event, but the sources are part of C::B.
wxAuiNotebook also does not provide an event, if the tab gets double-clicked, and we can not ship a patched version of wxWidgets with our sources.
That's why the patch never made it into trunk.
If we have updated the sources, I will have a look if it is possible to find an easy way to implement this feature, but I'm not sure whether it can be done with a reasonable effort.