A global pid is context information that probably shouldn't be shared across "context boxes".
You could possibly figure out what pid a "targeted" process is by knowing about when it
started and what processes started before and after. This would be useful in an attack
breaking confinement. I know this is all theoretical, and may not even be possible, but if
you can design it out you don't have to seriously think about it.
Why should processes in one box have any knowledge of the processes running in another
that don't explicitly announce themselves? If you are relying on this then you are really
in the same context (process namespace) anyway. It doesn't matter that the containers
are visible or not, this is one of the points in having containers with namespaces in
the first place, I would think.