We can use some sort of inheritance with HashMapObject by using #base.
I propose a "magic" HashMapObject special value #parent; a HashMap with which to gain access to an object's parent's methods:
private _baseClass = [
["updateMap", { /* parent class update code */ }]
];
private _childClass = [
["#base", _baseClass],
["updateMap", {
private _arguments = _this;
private _allParentMethods = _self get "#parent"; // HashMap
private _parentMethod = _allParentMethods get "updateMap"; // typeOf _parentMethod == "CODE"
// Call `_baseClass::updateMap`
_self call [_parentMethod, _arguments];
/* class specific additional code to "updateMap" */
}]
];