I added a comment in Using rcopy and sexp methods to create a .Call interface to a compiled Julia library · Issue #556 · JuliaInterop/RCall.jl · GitHub with a proof-of-concept for loading a shared library created by juliac into an R session and calling a Julia function from R.
This example uses the more basic .C interface from R, not the .Call or .External interface. I think it should be possible to extract the code defining methods for rcopy and sexp from the RCall.jl package into a separate package, say RObjects.jl, so that that code can be used in both RCall.jl and in code that will be compiled to a shared library to be called from R.
By the way, please comment in that issue if there are better ways of calling juliac, etc. for this usage. As I said, this is a proof-of-concept at this stage, not a demonstration that I know what I’m doing.