From what the description for the plugin http://txfx.net/code/wordpress/force-category-template says, this will do exactly what I need - carry the category template design through to a single post in a category.
I uploaded and activated the plugin, but when I try to look at a post in a category that has a special template the page gets stuck in a while loop. I get these errors:
Warning: rawurlencode() expects parameter 1 to be string, array given in /home/leadershipprogram/www/wp-blog-header.php on line 171
Warning: extract() [function.extract]: First argument should be an array in /home/leadershipprogram/www/wp-blog-header.php on line 184
Any other post not in a category that has a category template works fine.