AdditionalMultilist

using System;
using Sitecore.Shell.Applications.ContentEditor;

namespace CustomCode.Common.Fields
{
    public class AdditionalMultilist : MultilistEx
    {
        private const string Setting = "setting=";

        public new string Source
        {
            get
            {
                return base.Source;
            }
            set
            {
                if (!string.IsNullOrEmpty(value) &&
                    value.StartsWith(Setting, StringComparison.InvariantCultureIgnoreCase))
                {
                    var setting = value.Substring(Setting.Length, value.Length -Setting.Length);
                    base.Source = Sitecore.Configuration.Settings.GetSetting(setting);
                }
                else
                {
                    base.Source = value;
                }
            }
        }
    }
}

Patchconfig

<?xml version="1.0" encoding="utf-8" ?>
<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/" xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">
  <sitecore>
    <settings>
      <setting name="MySetting.HomePath" value="/sitecore/content/client1/Home" />
    </settings>
  </sitecore>
</configuration>