In principle, it works on Nvidia, but Nvidia still does not support 1.2... But, new pluggable engine for OpenCL 1.2 is in plan, and it will have optimizations for Nvidia, so, it is not something that is out of reach.

How about Intel/AMD on-die GPUs? (And any thoughts on if that's likely to help -- now, or with future generations -- as opposed to just sticking to the CPU side?)

So the library does not depend on OpenCL version or does?

The library does NOT depend on the version, since it supports pluggable engines. Currently the only available engine is focused on AMD GCN GPUs and uses OpenCL 2.0 features in its kernels. Other engines are possible and planned, notably one that would work with older versions of OpenCL and support tuning for Nvidia.

Going by the last paragraph in the article, the library currently supports "OpenCL 2.0" only, but "OpenCL 1.2" support is planned.