Pardon my ignorance, I know nothing about BIOS modding or SLI.
If part of the problem of getting external NVIDIA cards working is
-an SLI bridge soldered into the hardware (as David Washington suggests) and
-a conflict between different NVIDIA GPU models that prevents booting (as dsigned suggests)
would it be possible to blacklist the dGPUs (e.g. 650M, 750M, 755M) to avoid the SLI conflict? Or does the SLI hardware have some master/slave configuration that prevents this from working (i.e. booting with only the slave)?