Same issue — I never found a way around the clamp. I park the window's top-left at (maxX - 1, maxY - 1) of the union of all displays' CGDisplayBounds, i.e. just inside the bottom-right corner of the whole arrangement. The clamp is much looser going off the bottom/right than up/left — pushing to something like (-30000, -30000) gets snapped back near the top-left and leaves a full-height sliver, whereas bottom-right leaves just the corner pixel (plus its shadow, which I also haven't beaten).

Since the target is derived from the union, a display disconnect or resolution change shrinks the desktop and the parked window pops back into view. I listen for display-config changes and re-apply the position.