I am in a similar situation as you, but I got this to work. I have a Core i7 3770K and a Radeon HD7970. Using the latest available graphics drivers from Intel and AMD for Windows 8.1 64-bit, with my BIOS's "IGP Multi-monitor mode" set to "Enabled", I was able to use the IGP as my graphics card by plugging an HDMI HDTV into the motherboard, with nothing plugged into the Radeon, and use the Radeon for compute in the OpenCL application "Diablo Miner" (Java program). It lets you specify the OpenCL device, and both Intel and AMD ICDs were available, so I went ahead and chose AMD.
If you are unable to reproduce this functionality, it could be:
- Your IGP doesn't support multi-monitor mode? I believe this is required. What it should really say is "multi-graphics card mode" from what I can tell, but BIOSes are awful at describing what their options actually do.
- Windows 7, using an older revision of the WDDM specification, doesn't support the required infrastructure that's needed for this configuration? In that case, try Windows 8 or 8.1 (8.1 bumps the WDDM revision, so I'd go with 8.1 if I were upgrading from Windows 7.) Also of note is that AMD and Intel maintain separate drivers for Windows 7 and Windows 8/8.1, so it could be a Windows7-driver-only bug you are encountering.
- An OpenCL ICD issue? As I recall, older Intel (or was it AMD? I forget now) drivers have a problem with their OpenCL ICD, where the one "errant" ICD will take precedence over and hide all other ICDs from being made available to programs. If you're in this situation, you may have to completely uninstall and purge all graphics driver implementations on your system; ensure that no ICD remnants remain; and install fresh (as to what's the proper order of installing the Intel or AMD driver first, I couldn't tell you, as I'm not sure.)
In conclusion, there are various things it could be, but the configuration should be possible without having to create a fake VGA plug or anything silly like that. Recent Intel and AMD drivers -- at least on Windows 8/8.1, can't speak for 7 -- should be able to support "compute from anything, render to anything" configurations, where you can choose which device you want to be the graphics renderer, and choose which device you want to be the compute card. From what I understand, a chip on the motherboard from LucidLogix is supposed to support or enable this, but I don't have their software installed, so I'm not sure if my LucidLogix Virtu MVP device is enabling me to have this configuration, or if it's just built into the platform controller hub of all Z77-series motherboards.