I don't have any Mac with an AMD/ATI GPU, but if I'm not mistaken, unless you already have a partially working MacBook Pro 2011 only with the IntelHD3000, disabling the AMD/ATI through NVRAM command you could boot anyway.
Even if you succeed in replacing and soldering the new GPU CHIP, maybe could risk on the other side a kind of EFI CHIP Brick, I mean it couldn't pass the necessaries EFI (logic board components) CHECKS after chime sound, stucking on a black screen.
edit:
If you still can't neither power-on using the other IntelHD3000 GPU (because of faulted AMD/ATI GPU), then you have nothing to lose, and maybe your test could work being helpful for the others.
BUT seems exists also an "hardware way" to disable physically the AMD/ATI GPU on a dual GPUs MacBookPro, removing a resistor "R8911".
It seems also that in this way the machine EFI should be seen as having only the IntelHD3000, lets say as a MacBookPro 13" 2011.