You're absolutely right. If you force IGFX, & set the primary video device to IGFX, the board will disable the PCIe slot. Try this; set the primary video device to PCIe. Then set the IGFX to auto. This will turn on the PCIe slot and make it available. When the board doesn't find a graphics card it will jump to the IGFX while leaving the PCIe slot active.
I have the same board. There was some debate about this on hardforum and on the asus forums. This is the procedure I went through to use a wireless nic in the slot. Others have used ceton tuners in it as well this way.
Go into C:\\Program Files (x86)\\common files\\Terratec and copy the folder "Cinergy_T_PCIe_Dual" to the desktop.
Delete & uninstall all programs for the tuner. Make sure windows update is set to never check for updates. You want manual control over this. You want to make sure that windows update isn't installing a driver for the tuner. (I had this problem with an HD750, it borked it every time) Go into device manager, find the tuner & delete the driver. Reboot. Windows should find the tuner but no driver. Go into device manager & locate the tuner. Open the properties, click update driver. Point windows to the folder "Cinergy_T_PCIe_Dual" on your desktop. It should install the driver with no other tuner software. Then reboot.
Let us know if it works.