Additionally, a hardlink solution (as you point out) really doesn't sound too outrageous to me. A trivial tool to write to manage immutable things like binaries. Or is there some trouble I'm failing to see (possible)?
You're right. My personal reference for unionfs is NetBSD, which doesn't include such a warning [0]; apparently freebsd's implementation needs some love.
Edit: addendum
Additionally, a hardlink solution (as you point out) really doesn't sound too outrageous to me. A trivial tool to write to manage immutable things like binaries. Or is there some trouble I'm failing to see (possible)?