The way I understand the situation from the replys, Calibre's built in options do not provide menus. They may start to implement such, but not to the extent that they would be usable as one would expect.
And indeed my intent is to create a plugin that does provide full "classic" menus. The purpose of my post was to find out
a) if there already is such an effort and
b) if I overlooked some technical issue that would make this goal impractical to realize.
So far, I think the answers to this are No and No. Right? ;-)