@Herko-ter-Horst, No of course not! Although it seems that apps that do exactly that are available.
What I am proposing is the other way around: passing certain key presses of the foreground app (being MRA Mobile) to a secondary app. So if F3 is pushed (Barbuttons key 3 in keymap 3), it get's passed to Zello. In Zello you can define your own key that you want to use as PTT key.
By the way: it is already possible to use Zello as PTT application while navigating. You only have to use an overlay button on your screen. Tap once and the channel opens, tap again and the channel closes.
You can drag that button to anywhere on screen. You can even place multiple buttons to different people or groups. You can even define multiple hardware keys to multiple people or groups.
The volume slider is only visible because I miss-pressed the screenshot buttons 😉