Dynamically setting the IISUrl of a Web Application Project

Question

I have a web application project that I'd like to be able to branch and develop on a totally separate IIS site and not have to keep editing the vbproj IISUrl setting each time I do it. I have tried adding an import of some standard properties so I can maintain them outside of the WAP file but when I reference them in the element it doesn't like them. I suspect it's because the IISUrl element is part of which is a place to put non msbuild info.

As you can see in the code below, I'm trying to reference $(CustomUrl) in a couple spots in the WebApp1.vbproj but when I open VS it's not liking it.

Any way to resolve this or does anyone have a better way to branch off Web Application projects.

CustomImport.vbproj

<Project 
    xmlns="http://schemas.microsoft.com/developer/msbuild/2003"&gt;
    <PropertyGroup>
        <CustomUrl>http://localhost/WebUrl_1&lt;/CustomUrl&gt;
    </PropertyGroup>
</Project>

WebApp1.vbproj

<Import Project="CustomImport.vbproj"/>
<WebProjectProperties>
  <UseIIS>True</UseIIS>
  <AutoAssignPort>True</AutoAssignPort>
  <DevelopmentServerPort>1124</DevelopmentServerPort>
  <DevelopmentServerVPath>/</DevelopmentServerVPath>
  <IISUrl>$(CustomUrl)</IISUrl>
  <OverrideIISAppRootUrl>True</OverrideIISAppRootUrl>
  <IISAppRootUrl>$(CustomUrl)/</IISAppRootUrl>
  <NTLMAuthentication>False</NTLMAuthentication>
  <UseCustomServer>False</UseCustomServer>
  <CustomServerUrl>$(CustomUrl)</CustomServerUrl>
  <SaveServerSettingsInUserFile>False</SaveServerSettingsInUserFile>
</WebProjectProperties>