Through some script sleuthing, I did discover that Steam ships several of its own 32-bit and 64-bit libraries, and that paths to both are added to LD_LIBRARY_PATH (search path for library files) when the client is launched by the Steam Runtime, but many files (specifically the Steam Runtime) are only present as 32-bit binaries in ~/.local/share/Steam/ubuntu12_32.

Whether the Steam client uses those is a question someone else less sleep-deprived can answer.