There is a really quick way to unlock this trophy as you can repeatedly hijack the same vehicle and the game will count the hijacking towards the trophy. Simply get one of your gang members to sit in a police vehicle with you and you can repeatedly climb on the roof and then "hijack" the vehicle when your gang member takes over the driving.
Rinse and repeat 20 times for the trophy and 50 times for the "gone lawing" perk to unlock.
As per the comment below from Soniq, purchasing the upgrade that has your Rooks driving carriages might make this solution unworkable.