There is a page title override field in Page Settings.

Alternatively don't do that in the site title in Site Settings but instead just put the markup for your site title with the custom italics on the word directly in the layout.master instead of using the <portal:SiteTitle control.

For example on this site I have a different color for part of the word mojoPortal by wrapping a span around the Portal part of the word. But I did not do that from site settings, I simply removed the <portal;SiteTitle control and added this markup directly:

 <h1 class='siteheading'>
                <a class='siteheading' href='<%= Page.ResolveUrl("~/") %>'>mojo<span class="green">Portal</span><span
                    class="slogan">Advanced Web Sites Made Easy</span></a></h1>

Hope it helps,

Joe