Can a recommendation system be used as a binary classifier?
Would this approach work?


I don't know, you should try, it very well could.


Can recommender systems be used as classifiers?


Absolutely! But first, let's clarify a couple of things. Recommender systems are systems. They are the output you are trying to achieve. This can theoretically be achieved without machine learning but in general, building recommenders happens via a learning algorithm (i.e. collaborative filtering, neural nets, etc.).

What that learning algorithm does is learning to provide a "score" to each item given a specific context (i.e. user X wanted to watch a movie on a Tuesday night) such that the highest scores are given to items that are worth recommending.

This can directly be applied to a classification problem where your learning algorithm learns to give high scores to the correct class.

So in summary, you would still be building a classifier system (what it does) by using a recommendation learning algorithm (how it does it).