Thank you, your answer makes the case to be simply closed.

Don't give up that quickly :-) It's an old chipset and I you may just need to checkout coreboot-v1 (aka LinuxBIOS). SiS 635/735/950 were some of the first chipsets with coreboot support, thanks to a SiS engineer named Li-Ta "Ollie" Lo who eventually came to work for Ron at LANL.