Without server access I've never found anything that can protect directories like .htaccess can. However, there are some good php password protection scripts around. The trick is that you generally need to include something in every file to be protected. Depending on how many html files you have that could be trivial or it could be impossible.
Have a look at this script:
http://www.zubrag.com/scripts/password-protect.phpWhat you could do is simply rename all your .html files to .php and (after configuring it correctly, as per instructions on the website) add the requisite line into each of your pages to be protected. You'd also need to update any links to those html files to point to the php files instead.