There can be 2 issues here...
EDID of LCD connected to the 'thru' of the image pro is transmitted back to the mac. However, this can sometimes conflict with the programmed EDID of the Image Pro and you wont get an image on the 'thru' LCD
There is a known error with programming the EDID!!.. it requires a firmware update and an aditional manual command to be entered via telnet. (I have these details if required)
Are you sure its a DVI-A cable - not DVI-I... You should be able to use either DVI-I or DVI-D cable at this resolution to connect from the Mac to ImagePro.
(DVI-A cables are almost impossible to find and rarely used. If you really want analoge. Use a DVI-VGA adaptor or cable and connect the the imagepro analogue input.)
There a a couple of solutions:
1) Use a gefen DVI Detective on the Mac output to provide the EDID information - no matter what is connected after it. You should program this from the image pro - or whatever device gives you the correct resolution.
2) Use a Distribution amplifier between your mac and image pro. this will allow you to connect both LCD and image pro, and resolve issues of conflicting EDID on thru ouptut
3) Program the image Pro with the correct EDID information - this will ensure that you always get 1024x768 @ xHz
Let me know if you require instructions on how to update the imagepro if you are having issues programing the EDID.
For more info on DVI cables - check out the PDF attached.
Simon