I started working on such a script a few weeks ago, but never got around to finishing it. It is very similar to Sam's ALM in the sense that you can specify which liveries are applied to which vehicle, in which area, based on in-game areas/neighborhoods. I haven't done much testing, but it seemed to work with a couple of vehicles, so I guess you can say it's functional. But then I thought it would be weird to have LSPD officers get out of a patrol car with an LSSD livery, for instance, so I kinda put it on hold.
Now that I've secured myself a job, I might go back to it and implement a feature that lets you also specify which peds (and components?) are spawned in the vehicle, if the authors of this script don't beat me to it.
Sorry guys, I hope you'll excuse me for hijacking your topic.