I didn’t think RCall downloaded Julia for you, but I was mistaken, it does if you set R_HOME = “*”, using Conda. I’m not sure, that might still be at installation time, but since possible, could at least potentially be a runtime check.
About R packages themselves, I think they are usually downloaded as source and compiled, and arguably could also be done at runtime (Julia could have C++_compiler_JLL…, maybe does already, to provide all needed infrastructure, it’s just would you want to wait for compilation, and bundle it too or wait for it being downloaded, when first using?). I don’t know what package manager most R users use if any, but since R itself is provided by Conda, maybe at least some R packages are there also, precompiled?
I believe all (Julia) code can be supported (well one known exception, I forget the exact details), and my understanding is that there are no requirements.
Precompilation is on by default in packages (for a long time now), can be turned off (is it often done?), and I would have thought works either way. Because if PackageCompiler.jl wouldn’t (pre)compile all code (the main point), it still has LLVM at runtime and could compile. You can strip out LLVM, and then you would be more restricted. And since LLVM is big you might want to do that, but if you don’t care too much about non-small then not an issue. You will not get tiny binaries either way.
The GPL has been tested by a court, at least in Germany, and is being enforced in some casees (it’s never the GPL per se but for each specific GPL piece of code, e.g. busybox, and maybe nobody would bother for R or R packages). But if you don’t have an issue with the GPL/comply, e.g. provide a source with (for at least R packages), then I should be ok. Just note your full app will be “GPL as a whole”.