PSP heavily abstracts a lot of its hardware in OS/libraries. The GPU is not so heavily abstracted, why? Because low level access is important for performance. It's precisely because gaming devices aren't open that console companies are more willing to reveal the necessary low level information on how to program their GPUs, because they make game developers sign NDAs to receive this information. Most GPU manufacturers don't want everyone to know this information because they feel that it gives them a competitive disadvantage. Whether or not this is true is up for debate, but this is how the industry has usually worked without that much exception.
Even John Carmack admits that game consoles have a performance advantage over phones by providing lower level libraries. He says that PSVita will have a full generational advantage because of this.
Getting that kind of direct access to SGX w/o going through OpenGL ES 2 would require reverse engineering it and writing libraries for it (IMO, you would NOT want to just go and write your own OpenGL ES 2 implementation, or at least not exactly..). Someone on the old Pandora boards was doing just that but he appears to have vanished. No, it is not well documented, neither is any other OpenGL ES 2 level chipset I know of. In theory you'd probably have a much easier time doing low level stuff for Wiz/Caanoo because the Pollux datasheet has a fair amount of information (albeit in confusing English), but for some reason you don't see anyone attempting this either.
I bet, even the PSP Vita had to hide then behind the Pandora. ^^
