restrict access to everything except specified page(s) using .htaccess

I tend to upload 'work in progress' to subdomains of my site and use an htaccess file in the folder root to restrict access.

My client has a 'holding page' and wants me to transfer the site files to his server so he can work on the cms content, and then all i need to do is remove the htaccess restriction at go live.

my htaccess file to restrict all access is pretty simple,

# password-protect the directory in which this htaccess rule resides
AuthType basic
AuthName "This directory is protected"
AuthUserFile /path/to/password/file/
AuthGroupFile /dev/null
Require valid-user

How can i restrict access to everything except, say, index.html, so visitors can still access the landing page only ?

I googled a few 'solutions' to this, but none have worked.

As I can restrict access generally, I know the server is set up ok and the password file is in the right place,

instead of trying to restrict access to the site files, you should restrict access by IP address.

allow your client IP address to access the site, but show an under construction message to others.
http://www.inmotionhosting.com/support/website/how-to/redirect-visitors-under-construction-page

Is this solution compromised if the ip address is not fixed, which is often the case?

the htaccess file would need to be updated when the ip changes, but is much more complex to restrict access to only some pages using htaccess.