I'm having almost the exact same problem. It works fine with 8776, using the EDID information from my HDTV for the 1080i mode. I've tried tons of modelines that I've found on the net for 1080i, and they all result in displaying a blank screen.
All I want to do is to create a custom modeline that is the same as what it's getting from the EDID, so that with the 9xxx drivers it will work correctly. All my attempts have failed.
With the 9755 drivers, the EDID modeline displays 1920x540 instead of 1920x1080.
I'm beginning to think that there's a bug in the 8776 driver where interlaced modes from EDID are parsed differently from modes from custom modelines. Hence, it may be impossible in 8776 to specify a custom modeline that is exactly the same as an interlaced mode from the EDID. With the 9xxx drivers, the EDID is now interpreted the same way as the custom modelines, so while it's consistent now, there's no way to get it to display 1080i.