While this scenario isn't the smoothest if you have "View Admin Link" enabled, you can use the PHP method or shortcode and wrap them both in a link or button tag or whatever you want to code it. Here's an example:
<button class="btn"><?php add_modal_login_button(); ?></button>
In the future I am adding more ways for users to customize this better like adding custom classes to the the link so you can easily create a button or there will be default styled buttons that oyu can customize and so forth.
No set release date for that version, but I'm hoping I'll have time soon in the future