A theoretical question:

1. Would it be possible to "hijack" car radios temporarily by changing your PI code to a popular legal station's one (in theory car radios should then switch to your frequency at least in the transmitter's surrounding area, as your signal would be stronger than the bigger station's one here, if you have enough power).

2. ... to "hijack" car radios temporarily by sending out PTY-31/the alarm signal (thus they should all receive your signal). Is it still used in Europe?

I’ve found that not all car radios will switch automatically. It’s also not just the PI code but the “alt freq” also needs to be set and the radio goes for the strongest signal. I drive a Toyota Prius for work and there’s no option I’ve found to enable switching frequencies, however a Mercedes GLA series which my wife drives didn’t need any configuration and switches freqs no problem. We got to a point once on the road halfway between two cities and were in range of radio stations from both cities, the radio in the Merc kept switching between the two transmitters in both cities, the content was the same but different adds for the areas, one transmitter also had a delay of around a second.

A friend of mine was doing work on radio rebroadcast into tunnels, with "voice break-in" in case of emergency. They tried sending PTY31 and found that a lot of drivers would stop in the middle of the tunnel because "ALARM" had appeared on their car display, and they thought that it was the car itself reporting a malfunction!