I find the params option in js.Build more useful for passing in config etc.
As to your question, this is currently not possible. To pass data from templates to SCSS, the main.scss entry needs to be a template itself and built with ExecuteAsTemplate. I agree that this isn’t great, and we need to improve that.
I have a long list of things I want for Christmas, too. I doubt I get most of it. What you as for is not something you can expect anytime soon, if ever.
With the suggested approach above you can easily do:
.example {
background-image: $example_image;
}
Which would also be more idomatic Sass (if that’s even a thing).