gistfile1.cs

// in dialogviewcontroller.cs, line 370-ish

 public override UITableViewCell GetCell (UITableView tableView, MonoTouch.Foundation.NSIndexPath indexPath)
                        {
                                var section = Root.Sections [indexPath.Section];
                                var element = section.Elements [indexPath.Row];
                                
                                var cell = element.GetCell (tableView);
                                return section.CustomizeCell(cell, indexPath); // <-- added this
                        }

// in elements.cs, around line 2300, in public class Section
public virtual UITableViewCell CustomizeCell(UITableViewCell cell, NSIndexPath indexPath)
                {
                        return cell;
                }

// and my custom section! Obviously, you need the images, too.

using System;
using MonoTouch.Dialog;
using MonoTouch.UIKit;
using MicroAgent.Common;
using MicroAgent.Library.Database;
using System.Threading.Tasks;
using System.Drawing;

namespace MicroAgent.Library
{
        public class BTBackgroundImageSection : Section
        {
                
                public BTBackgroundImageSection(string header) : base(header){}
                public BTBackgroundImageSection(string header, string footer) : base(header, footer) {}
                public BTBackgroundImageSection() : base(null, BaseDialogViewController.MakeEmptySectionFooter()) {}
                public BTBackgroundImageSection(UIView header) : base(header)
                {
                        
                }
                
                public BTBackgroundImageSection(UIView header, UIView footer) : base(header, footer)
                {
                }
                
                public override UITableViewCell CustomizeCell (UITableViewCell cell, MonoTouch.Foundation.NSIndexPath indexPath)
                {
                        var theCell = base.CustomizeCell (cell, indexPath);
                        
                        if (!Util.IsIOS5OrBetter) return cell;
                        
                        if (Count == 1) //use one with top and bottom rounded
                        {
                                theCell.BackgroundView = new UIImageView(Resources.CellBackgroundFull.CreateResizableImage(new UIEdgeInsets(5,5,5,5)));
                                theCell.SelectedBackgroundView = new UIImageView(Resources.CellBackgroundFullActive); 
                                
                        } else if (indexPath.Row == 0) //top only
                        {
                                theCell.BackgroundView = new UIImageView(Resources.CellBackgroundTop.CreateResizableImage(new UIEdgeInsets(5,5,2,5)));
                                theCell.SelectedBackgroundView = new UIImageView(Resources.CellBackgroundTopActive); 
                                
                        } else if (indexPath.Row+1 == this.Count) // bottom only
                        {
                                theCell.BackgroundView = new UIImageView(Resources.CellBackgroundBottom.CreateResizableImage(new UIEdgeInsets(0,5,5,5)));
                                theCell.SelectedBackgroundView = new UIImageView(Resources.CellBackgroundBottomActive); 
                        } else //anything in the middle
                        {
                                theCell.BackgroundView = new UIImageView(Resources.CellBackgroundMiddle.CreateResizableImage(new UIEdgeInsets(2,5,2,5)));
                                theCell.SelectedBackgroundView = new UIImageView(Resources.CellBackgroundMiddleActive); 
                        }
                        return theCell;
                        
                }
        }
}