Do you remember about this issue: https://github.com/meteor/meteor/issues/6037 ?
I’ve played with something like this: https://github.com/juliancwirko/meteor-bootstrap-postcss-test
The example uses Bootstrap 4 with Scss from Npm package. I use PostCSS plugins to compile Scss files from Bootstrap. I don’t need fourseven:scss package here. And of course I can import .scss files from node_modules now.
You can go through the files to see how it works.
Unfortunatelly it won’t work with Foundation 6 (for now), this is because F6 uses complicated mixins structure and I can’t make it work with postcss-sassy-mixins plugin I’ll keep trying.
Fun and weird stuff but it works for Bootstrap and probably for many other libs out there