I am not a lawyer, but...
According to the research I have done, at least under US, EU and UK copyright law, it is NOT possible to claim copyright on two dimensional reproductions of two dimensional objects (not only mss but also paintings for example) which themselves are not subject to copyright.
What *can* be exterted is contact rights. So, if you order an image from someplace, and they make you sign a contact, then you are bound by the terms of that contact. However, if you for instance have access--however you have that access--to such a two dimensional image (see above) there is no issue of copyright.
What this means in practice is that if I were to take, for instance, the University of Washington Press volumes of Gandhari manuscripts and scan the images (NOT the text) there is nothing they could do about it.
Why do some museums and libraries try to restrict copying? Because they think they can make money, that's all! The Rijksmuseum some years ago considered just making high definition images of all its paintings free on line. Why did they not do it? the gift shop objected...
Jonathan