There's a system for more advanced collision check functions, it's used by the different ramp objects: the zcheck_script_bottom and zcheck_script_top variables. These are called with two arguments, the x and y position we're doing a collision check over, and should return the top/bottom Z value of the object at that point.
Pyramids are actually a pretty complicated shape (4 triangular regions with different slope) but I think the easiest way is to check the x and y position difference vs the centerpoint of the pyramid - e.g. if the x difference is larger than the y difference AND the point you check has a higher x value than the center of the pyramid, you're on the right-hand quadrant, so you'd lerp from the top Z value of the pyramid to the bottom Z value of the pyramid using the x difference divided by half the sprite_width of the pyramid object. (The left hand quadrant does the same calculation but mirrors the x difference, and the top and bottom uses the y difference and the sprite_height instead)