WriteObject can't be run in a thread other than Powershell's. I've got a situation where the Object i need to use in the WriteObject method must be accessed by the thread that created it, and it's not PowerShell.
WriteObject (Object-in-different-thread)
Any ideas?

Have you tried serialization? (If that's an option for you)

Nope. In my case target objects can't be extended or encapsulated in a serialized class.