need more info, i hate this part of waiting about to hear more about what you want to know about
Can I hack this box? Run another OS? Change the hardware? Install my own software? Use it to build a robot?
-Sure.
Devs don't have to use Direct3D for linux. Just use OpenCL which most newer GPUs support. OpenCL will be able to maximize more since Nvidia/AMD both support on the GPU side of things. I am fairly certain intel/AMD cpus also work with OpenCL so going to consoles and to PC would be easier and etc.
I don't know if that would all work but OpenCL is becoming more of a standard so i assume it would.
Not entirely sure you know what OpenCL is, it's used for GPGPU applications using compute shaders. Some games are utilising these more for complex computations that can be done in parallel.
OpenGL however is a cross-platform graphics API which is what you must be thinking of.