It isn't currently possible to use your own login HTML page on a hybrid application. However if you enable anonymous users you can create a login page in the modeller using the login widget from the AppStore. You can do everything you need within the modeller that you would want to do on a separate HTML page.
One of the improvements for Mx 6.1.0 is the following: "New page templates for login pages and wizards."
I haven't been able to test it myself, but it looks promising.
why don't you customize the existing login.html file to the login page you want it to be, but keep the basics of the login.html page. also customize the css file which is used by the login.html