Use DarwinDumper to dump your EDID.
Open EDID.bin with a hex editor.
Open EDID.txt to check your EDID.
First of replace the serial number with this one in the EDID.bin (find and replace).
06 10 12 a0 00 00 00 00 1c 16
This will give you the Apple required serial number.
Next replace the chroma info with this to get Apple style colors on your display.
6f b1 a7 55 4c 9e 25 0c 50 54
Last but not least you need to edit one of the descriptors with this one (probably not the first as this is most likely the detailed resolution, use one you don't need).
00 00 00 fc 00 69 4d 61 63 0a 20 20 20 20 20 20 20 20
This will give the iMac display data.
Save up the binary and leave it open.
Use edid-decode to check the modifications and look for the checksum.
Correct the checksum and save it up again.
Download iMac_Display_Override.zip below.
Edit DisplayProductID-717 with a plist editor (plistedit pro for example).
Replace IODisplayEDID Data field with the hex value of your new EDID.bin
Look for AppleDisplay (under display0 part under your GFX card).
Look up DisplayVendorID.
Rename the folder DisplayVendorID-756e6b6e to DisplayVendorID-<your_vendor>
Open the folder.
Switch back to IORegistryExplorer.
Look up DisplayProductID.
Rename the file DisplayProductID-717 to DisplayProductID-<your_product>
Install the folder in /System/Library/Displays/Overrides
If you did it right it will look like the picture below
Screenshot is made under VMWare Workstation running Mavericks (I didn't have my Mac around right now).
EDIT: DisplayOverride is not fully good, use the kext set available below (override download removed)