I have this old Mac Pro Workstation and can't get to the Boot Menu (tried a number of things including holding down Option), the screen only appears when the OS starts.
There is around a 2008 Workstation model if this helps. I think it's due to the graphics card I replaced a few years ago with a cheap £ 40 GPU on ebooks. Nothing shows before the operating system starts. I use for windows and usually want to use the graphics on board, but it doesn't seem to have anything on these for that reason?
I've taken pictures with some special Apple adapters or something I need, see here