Yes, this is possible too. Using a Hex Editor, edit the IGE file that belongs from the race you want to run. A way to do this, is using the DOS command FIND as follows
FIND /i "<full or part of the race name>" *.IGE
EXAMPLE: If you want to find what IGE file is from the "The forbidden fall" race:
FIND /i "the forbidden fall" *.IGE
FIND /i "forbidden" *.IGE
If you do it right, you gonna find the RSFE0022.IGE.
Now, with the IGE file founded, edit the following adresses:
0297 - Change to 00
02A7 - Change to 00
Now you can enter the race with any car/moto.
Oh, an important information.
Before entering the edited race, choose your car from your garage or a menu race that originally are NO RESTRICTION rules (Ex. "8 drivers make a record" or "Tour of island").
mod edit: Please stop double posting and use the edit button. Thanks!