Some other examples I can think of:
An annual race in Germany pitting F3 cars against lower classes such as Formula Ford or Formula Opel.
A Lancia Rallye 037 trying (and failing) to qualify for the 1994 Suzuka 1000. The same race saw cars ranging from a sole C-group Toyota 94C-V to a group of Honda Civic EG 6ixes.
The European GT4 Cup features the futuristic-looking Supersport class next to the GT4 regulars, like the Aston, the Corvette, or the Ginetta.
Weren't there instances of Mini Coopers starting in the World Sportscar Championship in the '70s? At Kyalami, for example...?
The various national GT championships tend to fill out their fields with what are basically touring cars, or even rally cars.
Monaco 1992 - an Andrea Moda starting together with the F1 cars

Across the Atlantic, there's the infamous 1969 Taladega 500 that saw NASCAR Grand Touring cars fill out the hole left by the boycott conducted by the majority of the Grand National regulars.
Jumping forward to 1981, the USAC Indycar Van Scoy Diamond Mines 500 with the participation of a group of USAC Silver Crown cars.
ARCA/USAC stock car driver Lee Raymond trying to qualify for an ASA stock car race in 1982 or so. The much heavier car stood no chance of making it in.
1993 (?) Copper World Classic Supermodified race at Phoenix - a sprint car starting from the back of the field, can't remember the name of the driver.
I look forward to your contributions
