I've managed to do this if anybody is interested. It involved the following steps:

1. Create class for custom data type. Rather than doing this in separate project I just did this in App_Code folder but you could do this in a separate project and copy the dll over. After doing this the field type will then appear in the Type drop down when adding a field in Contour.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using Umbraco.Forms.Core;
using Umbraco.Forms.Core.Controls;
using System.Web.UI.WebControls;

/// <summary>
/// Summary description for DatePickerPastDate_ContourFieldType
/// </summary>
public class DatePickerPastDate_ContourFieldType : FieldType
{
    public DatePicker dp;
    public List<object> _value;

    public override WebControl Editor
    {
      get
      {
        this.dp = new DatePicker();
        this.dp.ID = "dp" + (object) this.Id;
        if (this._value.Count > 0)
          this.dp.SelectedDate = new DateTime?((DateTime) this._value[0]);
        return (WebControl) this.dp;
      }
      set
      {
        base.Editor = value;
      }
    }

    public override List<object> Values
    {
      get
      {
        if (this.dp != null && this.dp.SelectedDate.HasValue)
        {
          this._value.Clear();
          this._value.Add((object) this.dp.SelectedDate);
        }
        return this._value;
      }
      set
      {
        this._value = value;
      }
    }

    public override bool SupportsRegex
    {
      get
      {
        return false;
      }
    }

    public DatePickerPastDate_ContourFieldType()
    {
      this.Id = new Guid("9e881b5f-7ca6-4567-9f2d-32f07b2b119d");
      this.Name = "DatePicker - past date";
      this.Description = "Renders a date picker that is restricted to date in the past";
      this.Icon = "datepicker.png";
      this.DataType = FieldDataType.DateTime;
      this.RequiresScriptManager = true;
      this._value = new List<object>();
    }

    public override string RenderPreview()
    {
      return "<input type=\"text\" class=\"textfield\" /> <img src=\"images/Calendar.png\" alt=\"datepicker\" />";
    }

    public override string RenderPreviewWithPrevalues(List<object> prevalues)
    {
      return this.RenderPreview();
    }

    public override List<object> ProcessValue(HttpContextBase httpContext)
    {
      List<object> list = new List<object>();
      DateTime result;
      if (httpContext.Request[this.AssociatedField.Id.ToString()] != null && DateTime.TryParse(httpContext.Request[this.AssociatedField.Id.ToString()], out result))
        list.Add((object) result);
      return list;
    }
}

2. Create the Razor script to render the field type. This needs to go in:  \umbraco\plugins\umbracoContour\Views\ along with the ones for other field types. The file needs to be called the same as your name in the class above without spaces etc. So my field was called FieldType.datepickerpastdate.cshtml.

3. Copy the contents from FieldType.DatePicker.cshtml however change the class attribute. This is used later to target the text box for the JQuery UI date picker. The source should be:

@model Umbraco.Forms.Mvc.Models.FieldViewModel        
<input type="text" name="@Model.Name" id="@Model.Id" class="datepickerfield_past" value="@Model.Value"
@{if (Model.Mandatory) {<text> data-val="true" data-val-required="@Model.RequiredErrorMessage"</text>}}
/

4. Edit the JQuery used by Contour to change the settings for the date picker. So as well as the standard date picker, we also have our new one in there setting the max date. This file is  \umbraco\plugins\umbracoContour\scripts\Frontend\contourform.js

        $(".datepickerfield").datepicker({ dateFormat: contourDateFormat });

        $(".datepickerfield_past").datepicker({ dateFormat: contourDateFormat, maxDate: '0' }); // our date picker

Thats it, all done. Hope this helps somebody. Contour V3 is proving to be very flexbile indeed!