As far as I know GPU emulation is easier than CPU emulation as the range of functions each component can perform is very limited with simple instructions. Especially as the RSX is a pre unified shader GPU, A modern GPGPU with unified shaders should be able to emulate any function on the GPU with a simple abstraction for most games (99% there will always be some game that does something really off the wall that won't work without the exact hardware) the only way to get 100% is to use the exact same hardware.
It is what they used for PS1 BC on the PS2 despite the GPUs being totally different and there is a list of PS1 games that didn't work http://en.wikipedia.org/wiki/List_of_PlayStation_games_incompatible_with_PlayStation_2, but most games work perfectly. Now modern GPUs are far more complex so there will probably be more games that won't work if they go this route next gen but the only alternative is putting a full RSX in the PS4 as Nvidia is at least 2 GPU architectures beyond the RSX anyway so they would have to use the same abstraction on a NVIDA GPU anyway.
Now the real problem is the CELL as there is a 0% chance of emulating that in the next 15 years so if they want BC they will need to use some form of CELL in the PS4.
P.S. One of the main reasons that PS2 GPU emulation couldn't be done on the PS3 was that the "Graphics Synthesizer" had embedded EDRAM frame-buffer (and a bunch of really unique memory management quirks) that had far lower latency and higher bandwidth than the VRAM used in the PS3 so there was no way of emulating the framebuffer accurately in software, PC emulators use a lot of tricks and buffers to get PS2 games to work but the emulation is nowhere near perfect and really wouldn't be worth it on the PS3 as it requires a lot of per game optimisations and and has a much lower compatibility rate, the PS3 uses a very standard GPU in comparison. There is a blog post about it for PCSX2 but I won't link that here for obvious reasons.
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