Just a peeve here - We had coders "tricking" GPUs into running semi-arbitrary code considerably faster than CPUs by writing it as custom shader routines, years before OpenCL hit the scene (Google for "GLSL" or "Cg").  So yes, the possibility does exist of mining on a completely non-CL-supporting platform.

That said, shaders (at least back then) had a fairly limited set of available operations they could do (mostly linear algebraic transforms on single or half precision floats, not bitwise ops on integers), and an incredibly small stack space (on the order of 256 bytes) - Good luck implementing SHA256 under those conditions.