Nope. I only mentioned the thread signaling in combination with the glFinish.
You can create a sync object, then "give" this sync object directly to another thread and this other thread with his own OpenGL Context can then wait/test for the sync object status.
EDIT: Just to include all details. You must create share OpenGL contexts to interchange anything between them. But most helper libraries, like for example SDL2, do this by default.
That is irrelevant though because the OP specified in #3 that they need the information between different processes, not just threads in the same process.