How do I make $.serialize() take into account those disabled :input elements?

Temporarily enable them.

var myform = $('#myform');

 // Find disabled inputs, and remove the "disabled" attribute
var disabled = myform.find(':input:disabled').removeAttr('disabled');

 // serialize the form
var serialized = myform.serialize();

 // re-disabled the set of inputs that you previously enabled
disabled.attr('disabled','disabled');

Use readonly inputs instead of disabled inputs:

<input name='hello_world' type='text' value='hello world' readonly />

This should get picked up by serialize().

@user113716 provided the core answer. My contribution here is just a jQuery nicety by adding a function to it:

/**
 * Alternative method to serialize a form with disabled inputs
 */
$.fn.serializeIncludeDisabled = function () {
    let disabled = this.find(":input:disabled").removeAttr("disabled");
    let serialized = this.serialize();
    disabled.attr("disabled", "disabled");
    return serialized;
};

Example usage:

$("form").serializeIncludeDisabled();

You can use a proxied function (it affects both $.serializeArray() and $.serialize()):

(function($){
    var proxy = $.fn.serializeArray;
    $.fn.serializeArray = function(){
        var inputs = this.find(':disabled');
        inputs.prop('disabled', false);
        var serialized = proxy.apply( this, arguments );
        inputs.prop('disabled', true);
        return serialized;
    };
})(jQuery);