So, I think I may have done this.  It seems to work anyway.  I'd be interested in comments -- did I do this right?

using System;
using System.Collections.Generic;
using System.Collections.Specialized;
using System.Linq;
using System.Web;
using EPiServer.Configuration;
using EPiServer.Globalization;
using EPiServer.Core;

namespace EPiServer
{
    public class ShortUrlRewriteProvider : EPiServer.Web.FriendlyUrlRewriteProvider
    {
        protected override bool ConvertToExternalInternal(UrlBuilder url, object internalObject, System.Text.Encoding toEncoding)
        {
            var pageRef = internalObject as PageReference;

            if (pageRef == null || pageRef == PageReference.EmptyReference)
            {
                return base.ConvertToInternalInternal(url, ref internalObject);
            }

            // Get the page
            var pageData = DataFactory.Instance.GetPage(pageRef);

            // Does this page have a simple address?
            if (pageData.Property["PageExternalURL"] != null && pageData.Property["PageExternalURL"].Value != null)
            {
                // "Reset" the path to be nothing but the raw site URL (so, essentially just the protocol and the domain)
                url.Uri = Settings.Instance.SiteUrl;

                // Now set the path to be just the simple URL
                url.Path = String.Concat("/", pageData.Property["PageExternalURL"].Value.ToString());
                
                return true;
            }
     
            // No simple address, just handle normally
            return base.ConvertToExternalInternal(url, internalObject, toEncoding);
        }

    }
}