I unfortunately don't know much about how the composer works. I don't think a composer.php file is required -- I believe that's only if you want a custom edit interface in composer. But I could be wrong -- maybe try experimenting with a composer.php file, and post back with any info you find out so we can help others in the future if they have the same problem.

-Jordan

I too would like to know how to get a custom 'Designer Content' block to appear and function correctly in the composer window. I tried duplicating the edit.php file and renamed it composer.php. The block actually appeared correctly in the composer window but unfortunately didn't work. It would be great if there was a solution to this.

This page appears to explain how to set up a composer editing interface:
http://www.concrete5.org/documentation/recorded-trainings/advanced-...

(Looks like you copy edit.php to composer.php, then use the $this->field('field_name') function to output the field names).