Did you ever figure out how to do this? I want to do the same thing.
Think about what you're doing when you're calling the ColdFusion.navigate() function - you're telling CF that you want to load in the contents of the target page (refresh_test.cfm) into your ajax-enabled DIV. Now take a closer look at the contents of your refresh_test.cfm page - notice how the page contains not only the contents of your DIV, but ALL the page content (including the <form> section that you don't want to refresh).
If you remove the content of your "refreshable" div to a separate CFM file, you can use the method you have outlined here to load that content in via CF's AJAX functionality without bringing in any of the additional content on your page.
1) Put code to be refreshed in a separate CFM
2) On your layout page include the code inside your <cfdiv> using <cfimport>
3) Use the ColdFusion.navigate function to refresh that <cfdiv>
That should get you to where you need to go.
Hope that helps!