Anyways, I don't know enough about the Windows installer to comment intelligently on what the issue might be, but I do know that the earlier drivers for HD 79xx did include OpenGL support and did seem to run OpenCL apps pretty well (at least to the extent I didn't see comments in the reviews).
You are correct; however, there is also a third driver out there, the "RC11" HD7900 preview driver, which iirc was the very first Catalyst for Windows release that supported the HD7xxx series at all. I am kind of wary about using this older driver (it's from January) because I am sure there are plenty of fixed issues between the 12.2 Pre-Certified Driver and the "RC11" driver which is basically a beta. The pre-certified driver, as I understand it, is currently undergoing validation and certification at WHQL, which is taking longer than expected because there are so many changes compared to the usual builds. I wonder if the Microsofties will notice the minor oversight that OpenCL is missing?