As far as I'm aware you'll have to change the source and recompile, dont think the menus.xml stuff touches the context menu. The main code for it is in src\ui\context-menu.cpp
Adding extra item related ones is pretty straight forward, just add another block like this to sp_item_menu:
/* Do something custom */
w = gtk_menu_item_new_with_mnemonic(_("_Do Something"));
gtk_object_set_data(GTK_OBJECT(w), "desktop", desktop);
gtk_signal_connect(GTK_OBJECT(w), "activate", GTK_SIGNAL_FUNC(sp_item_do_some_stuff), item);
gtk_widget_show(w);
gtk_menu_append(GTK_MENU(m), w);
then create the funtion sp_item_do_some_stuff that does whatever.
just add its declaration to the list above sp_item_menu
ie
static void sp_item_do_some_stuff(GtkMenuItem *menuitem, SPItem *item);
Hope that helps
Sim