It would be nice if there was a way to embed a custom element into the widget if you have paid for the Remove Branding plugin. This should be possible to implement securely by having an input on the Remove Branding settings that lets you set the content of a element. This would then be embedded at the beginning of the iframe content when the widget is loaded.
I understand you’d have to do your due diligence on that input to prevent XSS attacks, but it shouldn’t be too difficult, especially if you whitelist what CSS properties you allow.
This would be my preferred way of styling the widget to our brand rather than increased customization options. Fine tuned color control, padding/margin control over any element, and even use of a custom font via font-face and data-uri fonts would all be possible. It just needs to be served in the widget iframe from your servers