Sorry for such a delayed response.
The name of the variable implemented in intel/llvm is ONEAPI_DEVICE_SELECTOR (the name might need to be changed once upstreamed). It allows to select backends and device types with values like “opencl:gpu” and its syntax is described in detail here: https://github.com/intel/llvm/blob/sycl/sycl/doc/EnvironmentVariables.md#oneapi_device_selector
Yes, making it a top-level project makes sense to me.
I’m not aware of anything else besides that and __builtin_sycl_unique_stable_id, which is quite similar but provides a name for a global variable, which is used for specialization constants.