This is already quite possible! There are even multiple libraries to help you get started. Extending languages like this is a huge use case for Rust; one of the first production Rust uses was extending Ruby like this.

Hey Steve, glad to see you saw this post, now I get to personally thank you for all of your work on the Rust project. It's a really great community, it was very easy to get help in the IRC when I'd get stuck on new-to-me concepts. The documentation was also incredible, and the language itself is awesome! So thanks, and keep up the great work over there, I'm going to be definitely peddling Rust when I can haha.

Thanks! I have your paper open in a tag, im excited to dig in more. Congrats!