Given that ATI doesn't want people to flash the BIOSes of their cards, it is rather hard to get patched tables out to people. In that light, it is probably just Not Done to patch any tables.
One of the design criteria for AtomBIOS was the ability for the driver to over-ride any table with a newer version if required. We have been using AtomBIOS for the Windows drivers for a few years now and have never had to patch a table, but the mechanism is there.
It does restrict things which real code would not restrict. The choice between PAL or NTSC is largely dependant on what is available in AtomBIOS.
Actually, that's not the case with AtomBIOS. We don't restrict you to specific modes. There is a traditional VBE / BIOS layer running on top of AtomBIOS which does have a fixed mode table, but that only affects VBE calls.