Hey good start on the module. You’ve started out way ahead of me.
In order for your “app\design\adminhtml\default\default\layout\contentpage.xml “ to be activated, you have to turn it on in your module’s etc/config.xml. Otherwise, Magento won’t pick up the xml file at all. (make sure to turn off caching while you’re making your new module)
This will activate the parsing of your layout/contentpage.xml
The contents of your layout file should look like this:
<!-- this tag follows the URL pattern contentpage/main/ -->
<!-- put your block code under the "root" and replace whatever "content" block was there -->
<block type="contentpage/main" name="content" as="content"/>
Hopefully this should work. I haven’t tested it, but the idea is to make your block “contentpage/main” as the only block named “content” under the root. The only problem that you might have is if other modules want to add themselves to “content”, you will need to accommodate them as a block type “core/text_list”, which is the default type of the “content” block found in page.xml.