For good or bad, I spent 15 years on various SCCA National rules committees including a few on the SEB and 3 on the Solo Nationals Protest Committee.....
Even in the wild west ruleset of CAM, only an unmodified OEM spoiler would be allowed on a like car. E.G I could run the GT500 one you refer to on my '12 but not the new GT500 wing. The S550 guys were trying that wing, and the Camaro guys like the ZL-1 stuff but I don't think they have found it effective enough at low speeds, but I think it was determined legal.
The forward support would be considered an end plate (IMO) and not legal. They cover that in the interpretations.
Still no idea on the flap if it were part of the spoiler. That is a pretty interesting idea. I would need to get more front aero grip before I added more rear.
“#29454 Is a Gurney flap considered to be an end plate
As described a Gurney flap located at the top of a spoiler is not considered an end plate.”
This is in a 2021 SCCA bulletin (aggregate?)
Also found notes about strakes and vanes and such…okay if not more than 100sq in each and not closer than 6in. spacing. I’d go 10” high and 20” forward triangles.
Seems there are similar limits on splitter vertical elements.
Also spoilers can be as wide as the fenders/bodywork. 10” max curvature/arc. So make sure your design doesn’t exceed that either. At the angle I suggest, I don’t think that would be an issue.
So many rules and interpretations! I’m not 100% sure that applies for the OP, but worthwhile to check if you’re doing aero.
Hope that helps somebody.