Even though it's not according to the styleguide, you can just use snake_case whenever and wherever you want, consumers of your libraries or other way around will never notice!
It's very strange at first, and I had the same reaction as you, but it's actually quite nice. (I say that as somebody who strongly dislikes snake_case :) )
Or do you just mean use a mixture just like you could in most other languages?
Instead, you can import and use libraries that use a different stile without breaking your convention.