Get a reference to current Listitem from within custom SPField

Question

I've successfully created a custom field inheriting from SPFieldText and happily have complete control over rendering it on the input form as a control.

Problem:

I have a need to create a link to a popup with the ListId and ListitemID in the querystring when rendering the field using GetFieldValueAsHtml().

Something like this:

public class CustomField : SPFieldText
{
    public CustomField (SPFieldCollection fields, string fieldName)
        : base(fields, fieldName)
    {
    }

    public CustomField (SPFieldCollection fields, string typeName, string displayName)
        : base(fields, typeName, displayName)
    {
    }

    public override string GetFieldValueAsHtml(object value)
    {
        return string.Format(
            "javascript:window.open('{0}/_layouts/Popup.aspx?ListId={1}&ItemId={2}','Popup','status=0,scrollbars=0,titlebar=0,resizable=1,toolbar=0,location=0,width=600,height=500');return false;",
            SPContext.Current.Web.ServerRelativeUrl.TrimEnd('/'), 
            LISTID, LISTITEM.ID
            );
     }

Clearly SPContext doesn't hold a reference to the list or item and none of the properties seem to expose the current item. I tried overloading properties in the control but these don't seem to be invoked when rendering the field.

// None of these properties are invoked when rendering the field as above    
public class CustomFieldControl : TextField
{
     public override object ItemFieldValue
     public override object ListItemFieldValue
     public override string Text
     public override object Value
}

I've experimented with the RenderPattern in fldtypes_Custom.xml but again this is also ignored when rendering the field using GetFieldValueAsHtml();

Am I naively expecting something that's not possible? I'm open to any approach that avoids rewriting the web part... or just tell me it can't be done.

(The existing web part renders a grid and calls GetFieldValueAsHtml(). We know we can change the web part to achieve this but that's not an ideal solution for other reasons).