Alright, as a first set of changes, uploaded a ribbon bar fix to be able to focus on it with Alt.
So update to the latest version and here is a bit of an accessibility guide, once finalized it will be included in the support documentation:
Starting the tool, it will be focused on the Image page. It is used for adding targets for loading.
Pressing Alt will bring you to the Ribbon bar, use the arrows to move through it for the available options.
Use the key Up for main tabs, then Left-Right to move through those tabs if more than one, then Down to enter each of them.
All options in the toolbar have keytips, a letter that can quick-activate them, in case you find that useful.
If on an Image page, use Alt to go to the Add menu to add either an ISO, WIM or a directory, it opens a browse window which is part of Windows, as usual.
Selecting your image will add it to the Image page.
If focused on the ribbon bar, press Alt again, it will defocus the ribbon bar and focus back on the current page, again use the arrows to move through it.
Focus on an edition of the added image and press enter or right-click Load.
Tool will now read status bar actions as well, like reading data or completed.
On the Image page, pressing Tab will switch it between Images and Presets lists.
Once an image has been loaded, pressing CTRL+Tab moves you through the main pages on the left.
Pressing the Right arrow will focus on that page.
Apply page, ribbon Start will process the changes.
All this said, it won't be easy as the tool has a lot of text, grids and items, let me know of its accessibility downsides and we'll see how far can it be improved.