More than a year later, I decided to test one more time my old, dusty HD 7800 with the latest drivers and it seems that it almost works.
No more CL_INVALID_GL_OBJECT errors. Fabulous!
Just one GL_INVALID_OPERATION in the OpenGL code area, as discussed here: http://devgurus.amd.com/message/1284803.
Windows 7, 64bit
AMD Radeon HD 7800
Driver Version: 126.96.36.199 - Catalyst 13.4 (3-28-2013)
OpenCL 1.2 AMD-APP 1124.2