Can I configure one specific directory on my webserver to use an atypical default file such as "default.php" instead of "index.php"? Do I have to create a ".htaccess" file in that dir?
Yep, put the following in a .htaccess file:
And that will apply to that directory, and I think all subdirectories.
This assumes you're using Apache; other webservers may have other conventions.