Phil,
To do so now, the following code for a custom button should likely work for you. It can be placed anywhere in your code just like a normal link and you can use the image you'd like to use. This isn't part of the SCLogin module though. If you need how to modify that, I can try to help, but that would force any instances of the SCLogin module to go to this redirect location:
<a href="javascript:void(0)" onclick="jfbc.return_url=base64_encode('index.php?option=whereveryouwantogo');jfbc.login.login_custom();"><img src="http://cdn.sourcecoast.com/linktoimage.jpg" /></a>
Additionally, you will need the "Enable Redirection" setting disabled in the main component. If you've set a new user or logging in user redirection, that is always used first. That's why the option isn't in the SCLogin module, as it's generally easier to configure all your redirects for logging in and new users in one place.
As always, you'll need to test and verify that works as you expect.
As for when we'd implement the above, I'm not sure. It's something we get a request for here and there (not terribly often) and so we do have it on our to-do list. However, it will take a bit of work to implement and test across all cases, so we just haven't done it yet in lieu of higher priority tasks. As noted above, the different settings in the component would have to be implemented in the module, and it would get complex to make sure all the SCLogin modules are configured the same way (since that's how most users have seemed to like it).
Hope that explains and helps!
Alex