Very exciting to have the power of Faust DSP connected to a sample engine/ plugin generator/GUI powerhouse! Keep us posted, this is very interesting!
In regard to polyphony, with my experience its very easily implemented inside Faust code using
nvoices, however I don't know the exact way it works inside the compiler. Currently it's similar (in my eyes) to how csound does polyphony. (It "just works").
In regard to including Csound (and this is a cool idea) I feel that especially for physical modelling having access to the DSP code is much more flexible than using an opcode. Mind you currently Faust isn't the best for FFT resynthesis, (hopefully it will get that soon).
I'm not a Windows user, so can't comment on LLVM for that platform, mainly Linux, MacOS. (Addicted to POSIX).