Temporary files: RAM or disk?

A common cautionary tale template has an engineer confronting a problem and choosing a shortcut: "... and then the engineer had two problems". Unionfs has fundamental shortcomings that make it a problem where its usage pattern is not severely constrained.

How would this work, exactly? Would a daemon move big files out of tmpfs /tmp to a shadowed on-disk /tmp? This might steer clear of unionfs's shortcomings.