Hi, welcome!
I would make it a Feature request. It seems reasonable to allow HTML in this area.
It’s also possible to create a theme component. We can use a plugin outlet in this location. I quickly made one: GitHub - Arkshine/discourse-signup-cta-html. However, the preferred way would be to change the core as the components duplicate the code. (If you are new to theme component, you can follow Installing a theme or theme component)