Hi Sam,
That's a great idea.
In fact we're working on the next generation of web applications. In those you'll be able to have a different default button for each form you have on the screen. So if, for instance, you have a global search and a form, or a login form and a register form, in the same screen, the button that will be pressed when you click enter on a text input will be different. Expect more news about this soon.
As a workaround, for now, I think you can use different web blocks and each one of those can have a default button.
Thanks for your idea,
Tiago Simões