Hi, I have exactly the same configuration Acer Aspire 5742ZG. I installed niresh yosemite and went through the same installation process, used /s and grafix backup all to move the graphics kexts out, succeeded to boot into 1024x768.
As for drivers, I only managed to install nvidia CUDA drivers, but the CUDA interface in system preferences shows that nvidia gpu drivers are not installed. However, i couldn't get any of the current nvidia geforce mac web drivers to install (presumably because they aren't meant for a card as old as the gt 420m).
I also tried various bootflags but nothing has worked so far. Also set Graphics to 1366x768 in chameleon wizard, but nothing.
Finally, I tried to extract the GPU rom, for putting into the /Extra folder and boot with LoadVBios=YES (chameleon bootloader) but I couldn't get any version of GPU-Z (I have dual boot Windows 7 on another partition, and I run GPU-Z as administrator) to work. All GPU-Zs say that "BIOS reading is unsupported on this device".
I even tried dos booting with a usb stick to run various versions of nvflash --save xxxx_xxxx.rom but not even that worked. nvflash also said that everything was unsupported.
How did you manage to download the rom of this GPU? Did you finally manage to get the computer working in 1366x768 on Yosemite?