The libflowgpu that comes with the MVTools download does NOT work with the GTX 670 with any gpuid.
The libflowgpu that I downloaded from here http://www.svp-team.com/forum/viewtopic.php?id=697 (Probably the one that comes with SVPmark), works with gpuid=21.
So I presume the libflowgpu for MVTools has not been updated for the GTX 600 series because it was last updated 28.02.2012, which is before you patched in support in libflowgpu.
This brings up another question. Does libflowgpu not work with Intel's OpenCL implementation?