Hi mad,
someone who has experience porting chipsets could add support for the i965 chipset with maybe 6-12 months of fulltime work. This is of course under the (unlikely) assumption that full documentation is available. If you're doing it as a hobby and have to learn firmware development first, I expect 4 years of spare time, maybe more. After that porting to the Thinkpad x61 would be relatively easy.
For more info see http://www.coreboot.org/pipermail/coreboot/2011-July/065973.html
Regards, Carl-Daniel