New SteamOS Beta Has Non-UEFI Support
The SteamOS non-UEFI / legacy BIOS support is derived from the community's "Ye Olde SteamOSe" distribution fork. With this SteamOS Beta update, dual-boot and custom partitioning are also possible via the SteamOS installer's "expert install" mode.
While there's legacy BIOS support and early dual-boot support, Valve admits it's received little testing and isn't yet ready to be installed on production systems.
More details on this latest SteamOS beta can be found via this SteamCommunity.com announcement.