Create a computed observable that acts like an observableArray?

Currently I have a computed observable similar to:

// Backed with either an observable array or an observable of an array
var underlying = ko.observable([..]);

var obs = ko.computed({
   read: function () { return underlying(); },
   write: function (items) {
      // Process items - basically, I have the parent collection quickly
      // subscribe to an observable on each item. This in and of itself
      // should likely be cleaned up, but is not the focus of this question.
      // For instance:
      items.forEach(function (i) {
         if (!subscribed(i.title)) {
           i.title.subscribe(removeItemWhenEmptyTitle);
         }
      });

      underlying(items);
   }
});

However, I would like to be able to treat this computed observable like an observable array such that I can call obs.push(..) and such.

It's somewhat trivial to hack this up, but it doesn't feel right and I don't want to duplicate all of the existing observable array methods.

obs.push = function (item) {
  var arr = obs();
  arr.push(item);
  obs(arr);
});

Also, I might be missing a crucial difference between an observable array and an observable of an array.