Since there is no template for the subdomain index.html (will likely change), you can still create your own index.html files for them, but you have to use the subdomain_create_post.sh file.
Create:
/usr/local/directadmin/scripts/custom/subdomain_create_post.sh
In it, put something like:
#!/bin/sh INDEX=/home/$username/domains/$domain/public_html/$subdomain/index.html rm -f $INDEX cp /your/custom/index.html $INDEX chown $username:$username $INDEX exit 0;
Then chmod the subdomain_create_post.sh to 755. Obviosly, change /your/custom/index.html to whatever you'd like.