custom accel_key for a menu item

[gallows <g4ll0ws gmail com>]

Hi!
I've a Gtk::MenuItem, how can I apply to it a custom accelerator key?
I've tried this:

menu_item.set_accel_key(Gtk::AccelKey('z', Gdk::CONTROL_MASK));

But set_accel_key() is protected!
What is the right way to do this?

Thanks,

s.