Hideous hackish way to do this… You should be able to use System.Diagnostics.Process.Start in a customization. You can use Process caller in a configurator (for now…). Make a customization that simply opens a URL and closes itself. You can pass the URL via LaunchFormOptions I believe. Its gross, but I think it could work in a pinch. I think you should be able to hide the form during one of the startup events, but it might flash for a bit.
Edit: actually depending on what you want the URL to do, you might be able to open it directly with CEF in the customization.