At the moment this is not a built in feature, however with the addition of the 'PHP Block' in a recent release this is now possible with a bit of setting up:
Step 1: Create your page you want the user to be able to edit as the home page. Locate the ID number for this page. You can do this by going to the 'View Content' page and clicking on the edit link for the page. In the URL of your browser will be a section like: ...&id=23 where the number 23 is the ID for that page.
Step 2: Go to your site layout page and drag across a 'PHP Block' from the left and place it where you would like the home page content to appear.
Step 3: Enter the following code into the PHP Block, substituting the number 23 for the ID number for your page:
<?php
$homepage = iwp_content::getInstance();
$homepage->Load(23);
iwp_event::trigger(new iwp_event_content_afterdbload($homepage));
echo $homepage->Get('content');
?>
Step 4: Save your block, and then click 'Save Changes' for the layout. Viewing your home page you should see the content displayed. This home page is now modifiable by simply editing your page instead of the site layout.
