(Apologies if this is old news, it was new to me and I found it interesting)
From my recollection the main concept behind using 'remembered'/'spooshed' vehicles in stunts (thus being able to have a 3rd vehicle involved, for example) came from the fact that certain mission-specific vehicles were set to never despawn, right?
I was watching a Vice City speedrun the other day and noticed the use of a different way to 'remember' vehicles. It seems that if you shoot a car so the driver gets scared and hops out (hoping they aren't the ped type to stay in and just drive away instead), then you press F1 right as they are in the middle of that get-out animation, the game gets confused and never despawns the vehicle.
There are some quirks however - you cannot get in (or out!) of the vehicle via the door the ped used to get out from, so the best approach seems to be pulling up in your vehicle right up against their driver-side door before drive-by shooting at it, so they will be forced to get out their passenger side, and thus the car remains accessible to the player. You can even keep doing it and have quite a few remembered at once (I tested it out and had 4 going, in addition to my 2 normal vehicles), and it seems speedrunners may even make use of this to help clog up memory so there's less traffic on the roads.
I'm not entirely sure why I am posting this, I guess it's just cool to see another normal-game-behaviour way of achieving remembered cars as justification for using them via scms/mods. Plus this way is potentially even more flexible than the mission-specific vehicles approach, as it is pretty much open to any car (sadly it needs to have doors, so bikes are out for this) that peds can drive.