PS2 has modchips that have their own firmware already, so I wonder if one could be used to support a totally libre way to run both PS2 games and the Linux bootloader.
PS5 I'm not sure about, but I'd guess it's very very similar to the PS4 because it's just another glorified custom PC. If the GPU drivers work or only need slight modification, it'd be amazing to use with libre firmware cos it's a pretty powerful machine compared to the previous playstation consoles.