Calling UnitySendMessage from background thread

Is it possible to execute UnitySendMessage while being in background (e.g. by pressing Home button) and have corresponding Unity function called right away?

What I am experiencing is that my calls are stacked into the queue and executed one after another once app goes to foreground. I tried to use different background modes in xcode capabilities and also tried to set Application.runInBackground = true; but without any effect.

UnitySendMessage itself can be called from any thread, but it will invoke your script on main Unity thread. What exactly do you want to do on background thread?

I am working a plugin which listens to some system events when app is in background.
I want to send those to Unity app which might want to handle those in various ways, e.g. show local notification or save it somewhere (by executing HTTP request or locally in SQL database).
What are my alternatives here?

Same question for exact same reason!