You can accomplish what you are looking for without interacting with DNN at all by using an HttpModule. Kind of like this:
public class PidRewriteModule : System.Web.IHttpModule
{
public void Dispose()
{
}
public void Init(System.Web.HttpApplication context)
{
context.BeginRequest += new EventHandler(context_BeginRequest);
}
void context_BeginRequest(object sender, EventArgs e)
{
HttpApplication app = sender as HttpApplication;
if (app != null)
{
Match mPidCheck = new Regex(@"^/(?<pid>[0-9]+)/?$").Match(app.Context.Request.Url.AbsolutePath);
if (mPidCheck.Success)
{
app.Context.RewritePath("~/default.aspx", String.Empty, String.Concat("pid=", mPidCheck.Groups["pid"].Value));
}
}
else
return;
}
}
Then you can add this to your Web.config:
<modules runAllManagedModulesForAllRequests="true">
<add name="PidRewriteModule" type="Assembly.Namespace.PidRewriteModule, Assembly"/>
</modules>
Put that in the system.webServer node. Substitute Assembly and Namespace respectively.
All of this info is for IIS7. It's not entirely different for IIS 6, but previous implementations you have to go the route of ISAPI filters.