xBRZ is not GPU shader code. It's a scaler function and various utility functions implemented in C++.
There have been requests for a .fx (HLSL) port since 2012 on the DOSBox vogons forums, but no one has created one. Sure, it's possible to convert it to a shader - but not without some significant rewriting.
In any case, I agree that the xBRZ scaler output looks pretty good at upscaling resolution while preserving minor pixel details.