To set the crop based on pixels rather than a percentage, you only need to multiply the values by pixelWidth and pixelHeight. This converts from object-space to screen-space. If you do this, remember to change the parameters from "percent" to "float".
Now to set an angle is a lot more complicated. I think you will need to use the line equations to check at which side of the crop line the pixels are, based on the angle and distance from the center. If you do a naive check (to right of the line opaque and to left transparent) you will probably get a pixelated line. So to get a smooth line you will need to think in a mathematic function that returns a value between 0 and 1 depending on the distance of the pixel to the crop line, so you get an anti-aliasing.
There's probably other ways to achieve it. But either way, when you put angles and different coordinate spaces into the mix things get a lot harder.