Because DataIdMultiSelector is bound to the label of the type, you need to use a different widget to achieve your goal:

1. Create a file in the App_Code folder and add to it the following code that grabs IDs and titles of all C1 pages:
   

   using System;
   using System.Linq;
   using System.Collections.Generic;
   using Composite.Data;
   
   namespace CompositeGetData
   {
       public class GetC1Pages
       {
           public static Dictionary<Guid, String> GetC1PagesInfo()
           {
               using (DataConnection connection = new DataConnection())
               {
                   var C1PagesInfo = from dataItem in connection.Get<Composite.Data.Types.IPage>()
                                     select new { dataItem.Id, dataItem.Title };
   
                   Dictionary<Guid, String> C1Pages = new Dictionary<Guid, String>();
                   foreach (var info in C1PagesInfo)
                       C1Pages.Add(info.Id, info.Title);
   
                   return C1Pages;
               }
           }
       }
   }

   Download the source
   
2. Add a new C# function to C1 by registering it as CompositeGetData.GetC1Pages and selecting GetC1PagesInfo()
3. In the datatype's string field, set the appropriate length.
4. Switch to the Advanced tab and select a new widget: Composite.Widgets.String.Selector
5. Set the Selector parameters to the following values:

• Options: Execute complex function call => GetC1PagesInfo
• Multiple selection: Simple value => Yes 
• Compact Mode: Simple value => Yes