One very practical way to do this is to create a dummy start page for your tests that is simply a form with POST that has a single "start test" button and a bunch of <input type="hidden"
... elements with the appropriate post data.
For example you might create a SeleniumTestStart.html
page with these contents:
<body>
<form action="/index.php" method="post">
<input id="starttestbutton" type="submit" value="starttest"/>
<input type="hidden" name="stageid" value="stage-you-need-your-test-to-start-at"/>
</form>
</body>
In this example, index.php is where your normal web app is located.
The Selenium code at the start of your tests would then include:
open /SeleniumTestStart.html
clickAndWait starttestbutton
This is very similar to other mock and stub techniques used in automated testing. You are just mocking the entry point to the web app.
Obviously there are some limitations to this approach:
Please consider reading my article about the Qualities of an Ideal Test