Exactly what I did:
[*]Start up supermodel and cheat engine, wait for attract mode to start, then hook cheat engine to supermodel.exe and search for all values of type "byte" with the value of 0.
[*]Now, go into the service menu and under Game Assignments set the machine to be "MASTER". Select what car # you want to be here as well. Exit the the service menu.
[*]When you get the "NETWORK BOARD NOT PRESENT" screen, search for the value "1" and click "next scan".
[*]Repeat the above two steps with "SLAVE" and "LIVE" modes, with values 2 and 3 respectively.
[*]Now set the machine back to "SINGLE" mode, and scan for the value 0 again during attract mode.
[*]You should have 6 or 7 memory locations listed on the left side now. Check if any of them change, and double-click the ones that don't to put them in the table at the bottom, and check the box on the left of each one.
[*]Set them all to "1". This shouldn't hurt anything, and we're going to find the proper value in the next steps.
[*]The select screen should pop up normally, but the cars will have their multiplayer skins. Go ahead and choose a track and car and all that stuff.
[*]You'll notice that it'll get stuck at "Waiting for other players" after the transmission select. To fix this, go back to cheat engine and start setting the values you found to 0.
[*]When you hear the sound of a car being started, you've found the right one. Immediately set this bit back to 1, and you can uncheck or delete the rest.
[*]That's it! You should now be driving the multiplayer car that you selected in the service menu.
Yes it's a bit complicated, but something like this could be implemented as an emulator function or a proper cheat. Also, you'll have to do this every time you start the program since the memory addresses change for some reason.
Yes, thank you. I finally got it working now, to prove it, I got screenshots with the car I wanted.
Dustyjo's post should have a 50/50 chance of being sticked (the 50% of it not being sticked of people don't know what Cheat Engine and/or what Supermodel even is, and/or possibly have been confused in the steps he made. Thanks, Dustyjo!
EDIT (4/19/13): Looks like this thread's...dead? And I didn't get one reply... yet...