I'm not familiar with using javascript to force access to the site using HTPPS, but it would integrate with power store the same way it integrates into your regular site.
This does not seam to be a good way to do this to me though since javascript can be disabled in the clients browser.
to be honest, i have never dealt with a host that did not support forcing https using PHP or htaccess, I would be a little weary of this host.