select2_extended_ajax_adapter.js

$.fn.select2.amd.define('select2/data/extended-ajax',['./ajax','./tags','../utils','module','jquery'], function(AjaxAdapter, Tags, Utils, module, $){

  function ExtendedAjaxAdapter ($element,options) {
    //we need explicitly process minimumInputLength value 
    //to decide should we use AjaxAdapter or return defaultResults,
    //so it is impossible to use MinimumLength decorator here
    this.minimumInputLength = options.get('minimumInputLength');
    this.defaultResults     = options.get('defaultResults');

    ExtendedAjaxAdapter.__super__.constructor.call(this,$element,options);
  }

  Utils.Extend(ExtendedAjaxAdapter,AjaxAdapter);
  
  //override original query function to support default results
  var originQuery = AjaxAdapter.prototype.query;

  ExtendedAjaxAdapter.prototype.query = function (params, callback) {
    var defaultResults = (typeof this.defaultResults == 'function') ? this.defaultResults.call(this) : this.defaultResults;
    if (defaultResults && defaultResults.length && (!params.term || params.term.length < this.minimumInputLength)){
      var data = { results: defaultResults };
      var processedResults = this.processResults(data, params);
      callback(processedResults);
    }
    else {
      originQuery.call(this, params, callback);
    }
  };

  if (module.config().tags) {
    return Utils.Decorate(ExtendedAjaxAdapter, Tags);
  } else {
    return ExtendedAjaxAdapter;
  }
});

For anyone coming to this with the same issue I had:

when on some cases you don't have a list of default items to show. and you still want to display the default message "Please enter n or more characters", here's a small change to the query prototype in this adapter:

$.fn.select2.amd.define('select2/data/extended-ajax',['./ajax','./tags','../utils','module','jquery'], function(AjaxAdapter, Tags, Utils, module, $){

    function ExtendedAjaxAdapter ($element,options) {
        //we need explicitly process minimumInputLength value
        //to decide should we use AjaxAdapter or return defaultResults,
        //so it is impossible to use MinimumLength decorator here
        this.minimumInputLength = options.get('minimumInputLength');
        this.defaultResults     = options.get('defaultResults');

        ExtendedAjaxAdapter.__super__.constructor.call(this,$element,options);
    }

    Utils.Extend(ExtendedAjaxAdapter,AjaxAdapter);

    //override original query function to support default results
    var originQuery = AjaxAdapter.prototype.query;

    ExtendedAjaxAdapter.prototype.query = function (params, callback) {
        var defaultResults = (typeof this.defaultResults == 'function') ? this.defaultResults.call(this) : this.defaultResults;
        if (defaultResults && defaultResults.length && (!params.term || params.term.length < this.minimumInputLength)){
            var data = { results: defaultResults };
            var processedResults = this.processResults(data, params);
            callback(processedResults);
        } else if (params.term && params.term.length >= this.minimumInputLength) {
            originQuery.call(this, params, callback);
        } else {
            this.trigger('results:message', {
                message: 'inputTooShort',
                args: {
                    minimum: this.minimumInputLength,
                    input: '',
                    params: params
                }
            });
        }
    };

    if (module.config().tags) {
        return Utils.Decorate(ExtendedAjaxAdapter, Tags);
    } else {
        return ExtendedAjaxAdapter;
    }
});