There is a solution to this problem which I forgot when I last posted.
Basically instead of making a window of the exact size of the monitor you make it slightly larger, but then only draw to the part which overlaps your monitor. This ‘fools’ the driver into not engaging the fullscreen path and you can get a transparent overlay over the monitor. I tried this and it appears to work.
I multimonitor situations you might need to try not to overlap other monitors, though in some very rare cases this won’t be possible.