Page 46. it doesn't SAY that 256Mbit chips won't work.. it doesn't mention them at all. It also won't recognize them as 256Mbit chips. It recognizes them as 128Mbit chips.
If you'd like to test this, purchase a 128MB sodimm that has only 4 chips on it, put it in the IO and watch it detect 64MB. Then (in Real Mode DOS) run a program that reconfigures PCI bus 0 device 0 offsets 58 - 5F with values that you believe will configure the memory controller properly, write to memory addresses in that region, read it back and verify it.
If you perform this test and are successful, let us all know, and we'll find some way to get the better configuration into the bios.