Add site root token to lookup field queries

Multisite.ReplaceSiteRootToken.config

<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/">
  <sitecore>
    <settings>
      <setting name="MyProject.Multisite.SiteRootTemplateId" value="{161227F1-2BC1-4346-B833-D9DBED0B7104}"/>
      <setting name="MyProject.Multisite.SiteRootToken" value="$siteRoot"/>
    </settings>
    <pipelines>
      <getLookupSourceItems>
          <processor type="MyProject.Multisite.GetLookupSourceItems.ReplaceSiteRootToken, MyProject"
                   patch:before="processor[1]" />
      </getLookupSourceItems>
    </pipelines>
  </sitecore>
</configuration>

ReplaceSiteRootToken.cs

/// <summary>
/// Will replace a site root token in datasource queries of lookup fields.
/// Example: DataSource=$siteRoot/Home will be replaced to the actual site root relative to the context item i.e. DataSource=/sitecore/content/site_xy/Home
/// </summary>
public class ReplaceSiteRootToken
{
    public void Process(GetLookupSourceItemsArgs args)
    {
        Assert.ArgumentNotNull(args, "args");

        if (string.IsNullOrEmpty(args.Source) || args.Item == null)
        {
            return;
        }

        var siteRootTemplate = Settings.GetSetting("MyProject.Multisite.SiteRootTemplateId");
        var siteRootToken = Settings.GetSetting("MyProject.Multisite.SiteRootToken");
        if (string.IsNullOrEmpty(siteRootTemplate) || string.IsNullOrEmpty(siteRootToken))
        {
            return;
        }

        var rootItem = args.Item.Axes.GetAncestors().FirstOrDefault(a => a.TemplateID == ID.Parse(siteRootTemplate));
        if (rootItem == null)
        {
            rootItem = args.Item;
        }

        // Replace the site root token in the query or path
        args.Source = args.Source.Replace(siteRootToken, rootItem.Paths.FullPath);
    }
}