easiest thing i think would be just compile MESS to run on top of a kernel developed for your custom hardware? I know there are CPUs out there such as the ARM or MIPS type that have loads and loads of linux/unix kernels made for them, compile MESS on top of that, with the drivers written and linked into the kernel for your external devices, and video device. theres your mac. Thats how modern LCD/Plasma TVs are done. They use a CPLD/FPGA with an on-chip CPU (thinking of MiTek). all in one for the panel timings and etc.. running an embedded linux environment. Philips did exactly this. RGB LED signs are done in this fashon as well. FPGAs handle the LED timing, sustain, and color depth while the CPU runs all the scalar code and etc.