Hi Niels,
Ok, well first things first - I've added a template option so you can set an expression which sets the button disabled or not.
The createButton method supports a "disabled" parameter, so it makes sense to expose that as a template field.
That'll be in the next build.
In terms of adding "free form" extra html inside the tag, many form fields (although not buttons) support the loc:extra approach. I'll add that to buttons as well, although that may take a bit longer.
cheers
Bruce