I stumbled across the following forum post yesterday (http://www.insanelym...howtopic=192792) and, while it was for the x1350, I attempted the same steps applied to the x1400...and it worked!
So this is what I did. I've tested this with 10.5.7 iAtkos and iDeneb versions. My laptop is an Acer 5672.
1. Download the ATI_Kexts.zip file from here or the aforementioned forum post and extract it.
2. Using any text editor, edit the info.plist file from ATIRadeonX1000.kext and change the following string: from 0x71961002 to 0x71451002 (because the dev id for the Mobility x1400 is 7145. It's 7196 for the x1350.
3. Download a freeware program called softMCCS. This is used to extract your EDID information for placement inside the RadeonHD.kext info.plist.
4. Once downloaded, launch softMCCS and go to file->save EDID as->choose file type "viewsonic EDID file"->save your edid.dat.
5. Open this edid.dat file with any text editor, and remove all the white space from the beginning of the file, and remove the headers and columns (so essentially, you're left with rows and columns of xx data). Now format all of these xx data bits into groups of 8 followed by a space, so you basically have 32 chunks of xxxxxxxx data (xxxxxxxx xxxxxxxx etc...). See the attached files for my modified versions. Heck, these files might even work for you.
6. Now that we've got our modified .dat file, copy and paste all of the 32 chunks, and paste them into the RadeonHD.kext info.plist file, under the section <key>EDID</key><data>
7. Install either flavour of OSX (might work with other versions but I haven't tested this yet). Using the free program KextHelper, first install your modified ATIRadeonX1000.kext, repair disk permissions using Disk Utility, and reboot.
8. Again using KextHelper, then install your modified RadeonHD.kext, repair disk permissions, and reboot. If you get a kernel panic, reboot using -v -f commands.
9. If all boots well, you should now be able to change your screen resolution to 1280 x 800 with QE/CI.
The only problem I have now is mouse tearing. But I'm sure that's an easy fix. I just haven't looked into it.
Let me know how it goes for you!
UPDATE: I've created a PDF guide with images for the above steps. Let me know what you think!