Well, organics don't need as rigid a perspective as architectural does. If you screw it up a little, it won't look *off* right away. For organic blocking in sketchup, you might simply block in the position and scale of the person via a stand in 2D person... or the size and rough shape of the creature.
That would take me 2 minutes to block up and export, including the time to open up the program and export the .jpg
I'm not talking about a complicated underlay; literally extruded cubes with maybe a simple profile cut into it. Given the similarity of the cars you've drawn, you could literally use the same underlay for all of them.
*edit* here's what I'm talking about.
It's especially a big time saver for architecturals. Indeed, I combine a sketchy overlay style with 3D base building renders for my own design work to great effect.