Several full time mesa developers are paid by companies that have proprietary drivers running on Mesa in Windows. Which is why they take care to make sure Mesa compiles in Visual Studio, maintain the scons build system, etc.
So yes, there is, although you won't find any of them on this website.
Though i don't think that's any reason that TGSI has to stick around. They just modelled it off that as something easy to start with that they knew would work. I'm sure they could move to GLSL IR eventually if they wanted to (which may not be very likely - i'm guessing Intel would have to make a commitment to move their drivers into Gallium before it would happen).
I am not by any kind a GPU driver programmer,...yet, but I think that DX front-end(HLSL,runtime, etc.) is written by m$ and is closed source and in the end it spits something which can easily converted to TGSI, if not the same. Maybe someone from VMware can explain better.