Have you tried creating your own DTD for that mode (assuming the chipset dot clock will go that low) then setting the driver to only use your DTD and set "fixed timing" in the port screen? I believe that will force the driver to use that mode even when the driver is told to use Mode 3.
Additionally, you can tell the VBIOS to set your special mode (0x121 I believe) for boot and that should make that mode be your primary mode,
The only problem MAY be the use of mode 7 for text as that is 700 x 480 which will not fit within the display you want to set.
We do have ISV's that are authorized to work on IEGD code which we could get you in touch with if coding is required to support this capablity.
For a custom DTD, the VBIOS creates a custom set of VESA modes starting at 0x120 which is the DTD resolution at 4 bits per pixel, 0x121 is 8bpp and 0x122 is 16bpp