I don't think that is a big issue at this time. The Rust code would be building a data model in memory, one could easily do an internal DSL or call it from an external language.
Exactly. All a model does, is build a C-ABI-compatible data structure. And even though Rust is currently the only language supported, Fornjot is architected to be language-agnostic.
I don't think Rust is the ideal language for this problem, but with Fornjot itself written in Rust, it was the easiest one to support. Gotta start somewhere.