There is an exception.
With DBPro you can copy the image to a new image number and the memblock conversion will proceed without error.
When you attempt to do this with DBPro, you end up with an image lock error.
So you can paste the image to screen, and then Get Image to convert the image to a new separate image number (which isn't locked by camera drawing).
This then means that the image has been copied to a new image number and conversion to Memblock will succeed. The downside is that you had to paste the image to screen.
So if you switch from Bitmap 0 to Bitmap 1, before this process and switch back to Bitmap 0 after, you will not see the image pasted to screen and you will achieve the same effect.
You take some performance hit doing this. However this is a solution to converting a camera image to memblock.
Reference:
https://forum.thegamecreators.com/thread/184514
This is the Transparent Screenshot method I mentioned. The code could use some cleaning up. Instead of swapping bitmaps to hide the effect, it just copies the existing screen and pastes it over the image to hide it. However it demonstrates copying image contents into a separate image number to allow the memblock conversion process.