[are you saying that I can create an arbitrary triangle out of the ramps surface to do this check?]

It's no so much you're creating a triangle, you're creating a plane. Although you're using 3 points to determining the ABCD of the plane, the triangle is just a subset of the points that satisfy the ABCD equation. Use any 3 points on the plane that you wish.

[Or what point to choose on the triangle to make the vector]

Pick any point on the triangle. Or get fancy, and pick any point on the plane. You'll still get the same projection (with exact arithmetic). I picked p0prime, but you could really pick any convex summation of p0prime, p1prime and p2prime.

[aren't you in effect suggesting winding, too?]

Winding works too. In my codes, I use this solution because the backface culling step is done before perspective projection into screen space.