Then what I said was one way to do it:
Start in one terminal window mpv in fullscreen mode (trivial), then start another terminal window, borderless if you like (xterm in fullscreen mode for example), with a transparent background over it. You may use a short sleep between those actions, so the second does not become the first.
Depening on the WM/DE you're using, transparency is given (xterm cannot do transparency; if you use xterm, you need the WM to make the window tranparent, or use antother terminal emulator that provides it.) So check the DE/WM's manual how to open self defined windows.
If not, transparency can be reralized easily with an additional program install for X. I played a bit with it a couple of years ago, works pretty fine, easy to use, but alas I forgot its name, since I don't use transparency myself: It looks cool, but it highly confuses me; personally I simply cannot work on text, especially not code, when the background ain't monotonic, but being a picture - especially a moving picture was impossible for me to do any efficient writing over it.