Novice Mod Dev here, still developing Mods that I only use by myself, but if I had to guess, this wouldn't be OVERLY complex but it would certainly be a bit of a Pain to make happen. You would essentially have to generate the PED to where it actually exists, (parked cars with no PEDs just use whatever name was generated for the car when it was spawned into the World, the PED doesn't necessarily actually exist in the World) and then specify where that PED Spawns, how the interaction should go, create multiple spawn points (pretty easy), ect ect.
The main thing would be generating a PED with the exact same attributes as the PED that may come up when you run a Car Plate, e.g. if you use something like Compulite, the PED that gets generated may look different than the PED that the Car comes back to, have different attributes (e.g. DOB, Warrants, Arrest History, Address, ect), that sort of thing.
So, would making a Caveman version of this where you throw Immersion and continuity out of the Window be simple? maybe, but making something like this that would actually be immersive and fully functional? taller order. Ideally you would create a Callout for it that incorporates the Car itself, that way the Car is Spawned by the Callout, knows exactly where to take the player, ect.
I could be wrong, I could be sort of right, but from my relatively limited experience developing Callouts, this would be a bit tedious to make happen. For me personally, if I find a Car Parked Illegally, I just run the Plate, get the name, and give the "Owner" a Citation which will pop up in my System a few days with a Verdict.