Oblivious question: is there any notion of granulated permissions, ie a ‘mouse’ user group that the game could add itself to? Seems like something like this shouldn’t be a deal-breaker.
There is an 'input' user group, which gives you access to raw mouse, keyboard, joystick, et c. under `/dev/input/`. (I'm sure you could configure udev to use more more granular user groups for different types of input devices, if you really wanted). Normal human users of a single-user system should be members of the 'input' group, so this "should" be a non-issue.
I have added support for reading the input device directly on my 3D game engine exactly so i can provide unfiltered mouse input, but at least on Debian (and i think on derivatives) the user isn't on the 'input' group by default. The engine shows a message if raw input is enabled it cannot open the device handle (something like "cannot open handle check if you have permissions for accessing - maybe you need to be in the input group?") and it falls back to regular X11 events (the feeling of which depend greatly on the current configuration, which input driver is used, etc - on my PC where i use udev and 1:1 mapping it feels fine, but others use libinput, which i think is the default nowadays, and mappings can be all over the place).