Can I explicitly specify the NavigateUrl on a hyperlink?

Can't you simply use a HTML anchor (without the runat="server" attribute)? E.g:

<a href="relative.htm">link text</a>

Update: if you don't want to lose the functionality of the HyperLink control, you could create a control deriving from HyperLink and override the AddAttributesToRender() method (this is where the NavigateUrl is resolved).

HyperLink.AddAttributesToRender() looks like this:

protected override void AddAttributesToRender(HtmlTextWriter writer)
{
    ...
    string navigateUrl = this.NavigateUrl;
    if (navigateUrl.Length > 0 && base.IsEnabled)
    {
        string str = base.ResolveClientUrl(navigateUrl);
        writer.AddAttribute(HtmlTextWriterAttribute.Href, str);
    }
    ...
}

Custom HyperLink control:

public class MyHyperLink : HyperLink
{
  protected override void AddAttributesToRender(HtmlTextWriter writer)
  {
    if ((base.Enabled && !base.IsEnabled) && base.SupportsDisabledAttribute)
    {
      writer.AddAttribute(HtmlTextWriterAttribute.Disabled, "disabled");
    }
    base.AddAttributesToRender(writer);
    if (this.NavigateUrl.Length > 0 && base.IsEnabled)
    {
      writer.AddAttribute(HtmlTextWriterAttribute.Href, this.NavigateUrl);
    }
    if (this.Target.Length > 0)
    {
      writer.AddAttribute(HtmlTextWriterAttribute.Target, this.Target);
    }
  }
}