Hi Iņigo,
I was inspired by you and also started looking at adding a menu to fbreader.0.10.7. I used some code from xournal and had basically reached the same level as you have. (It was late last night so I decided so send you my progress today, seems we have been working on the same thing last night.)
I also looked at how to trigger the real functionality. It seems that ZLApplication::doAction(const std::string &actionId) in \zlibrary\core\src\application\ZLApplication.cpp should do the trick, but I did not have time to try it yet. It is also used by the toolbar and menu code.
Marcel.
(Next step would be to make the fbreader-DR-menu configurable with an xml-file

like all the other stuff in fbreader.)