After struggling with this for an afternoon, I hit on a fairly simple way to duplicate and modified master page file, since RH8 doesn't seem to have a method for copying them, and editing certain sections is difficult.
In the Windows Explorer view (ie. NOT in the RH interface) create a copy of the file you want to base your new Master Page file from. If you are using source control, you'll need to make the copy read/write as well.
Rename the copy.
Open the copy in a text editor and change the topic title in both the <meta name="topic-comment" content="topic title" /> line and the <title> line. You should also change the reference to any icon art you may use. Save the new file.
Open the project in RoboHelp.
In the Project Set-up pod right click on Master Pages and click Import. Select your new master page and import it.