I sometimes fantasize about rewriting Ewig in Rust, as a learning project (have been reading about it but didn't dare to write code in it yet). But I'd like to use RRB-Vector there too. Sadly, this data-structure is not really trivial to implement... (the concat algorithm in particular). Maybe one day I'll find the time :-)
Fortunately, a few months ago, after another attempt at Rust writing a simple ML-like language, something clicked and now I'm enjoying writing Rust. If you do attempt Rust, just expect to be frustrated for a while with Rust's ownership system, it's a new concept for almost everyone. I think it's kind of like monads in Haskell, you struggle with it until suddenly it "clicks" and then it doesn't seem very hard, but it then becomes hard to convey your understanding to someone else.
Rewriting Ewig in Rust, including the persistent tree-based immutable vector, would be an excellent exercise. Indeed, just starting with the RRB-Vector would be a great project. By the way, your immutable data structures library is quite impressive, it would be nice to have such a library in Rust. As far as I know, there is no such maintained library of functional data structures in Rust.