One option is to use kadira:dochead.
It allows you to set header tags via javascript. What this means is that you could put your dochead code (which is dead simple) inside your router. Just put your CSS files in your public folder so they are excluded from the default build process.
Here is an example of how I use it to load the dropbox password strength meter javascript only on the signup page.
FlowRouter.route('/signup', {
action() {
//Load password strength script only on signup page
DocHead.loadScript("https://cdnjs.cloudflare.com/ajax/libs/zxcvbn/4.2.0/zxcvbn.js");
//If the user is not signed-in, show this
if (!Meteor.user()) {
ReactLayout.render(MainLayout, {content: <SignUp />});
}
}
});