I think this will be a pretty easy addition/change:
Thanks for sharing your idea, I appreciate it. The Layout.ini file will usually contain only paths to files on the system disk, but it is entirely possible (I have seen it) that there are paths to other disks. It incorrect to assume that all the paths in the layout.ini file point to files on the disk where the layout.ini was found. Another problem is that a disk can have more than one layout.ini file, from multiboot installations.
But if there is a way to do this using scripts, I'd be more than happy
See the ImportListFromFile
fileboolean. It can import a layout.ini file from anywhere you want. It cannot replace driveletters, you would have to do that outside MyDefrag, for example with Jonib's tool.