Doing this properly requires very heavy math. I do it for the "expressions instead of keyframes" option of MochaImport+.
It allows to move and even keyframe the corner pin corners freely and still make them move with the track automatically. But it requires some heavy preprocessing of the tracking data inside of MochaImport+ before the tracking data is generated.
See this tutorial to see it in action:
So if your corner pin comes from a mocha track, MochaImport+ is a good solution. Otherwise it will be very tricky.
Well the reason your current code referenced above doesn't work is because you have "+ *" on lines 23 & 24, these should only be one or the other.
If it's a static amount of offset, why not just use this:
value + [20, 50]
If I understood it correctly, the offset should not be static in comp space, but adjust according to the movement of the corner pin. Like a static offset in the space defined by the corner pin surface.