I just posted a SteamOS ISO that can be used to install SteamOS on non-UEFI systems. Thanks to directhex and ecliptik for their work on Ye Olde SteamOSe[github.com] - this incorporates many of their changes. Dual-boot and custom partitioning are now possible from the "Expert Install" option.
PLEASE note there has been very little testing on this, especially any kind of dual-boot setup. So don't install it on any machine you are not prepared to lose.
If you have any problems, please post on this discussion or open an issue on the SteamOS github.[github.com]