Re: "only nintendo" you all probably know as well as I more than a few emulator authors have had a knock at the door or when "classic compilations" come out have cause to raise an eyebrow towards the developer. What I will note is between game source and possibly a bit of recompilation you can make a pretty good game specific emulator.
Short version- Janthran you have kicked off/found yourself in a kind of "well in theory....." thread as opposed to those wishing to get things done/create something you can run as it would on the original hardware.
To this end you should probably note emulation (especially of custom embedded hardware on radically different hardware) is hard area of programming, programming for embedded devices is nicer but still requires a bit of thought and emulators written to run on said embedded devices is often the realm of a masochist and doubly so when pushing things as hard as a GBA emulator does. Short of the options mentioned already nobody is holding their breath for a gaming grade emulator in the near future or at any predictable point in the future. Nice tech demos, ones you can try to wrap around certain games and modify to work (game specific emulators- there was a nice one for international karate in the early days of the DS if you fancy reading up on the concept) and one you could use to pull apart a game (not that you would really want to when you can just do it on a PC) sure but not gaming grade.
Back to your most recent question theoretically yes (and it will also require unsigned code) but the key part of that sentence was the one that dealt with shortcuts in emulation. Ignoring the very real issue of hardware specifics (only working for a certain save type or something) which are not necessarily a simple hack to add (back) in for various reasons consoles can have many modes of operation and for a truly accurate emulator (which some games require) you get to emulate each option with the timings it had on the original hardware. Somewhat elegant solutions to problems can be achieved when you know the hardware; for instance if something has not completed on another core by a certain point you can tell the game to jump somewhere else but if your emulation of the other core runs at 50x the original speed under the assumption of "who cares" it will almost be assured to have been completed and the failure to complete a task will never happen when the game expects it and it will never jump where it is supposed to go and thus your emulation can be considered to have failed. Now not all games will use such techniques (some are very easy on the hardware but that is OK as you can make great games using simple methods) and these are the ones that work which is enough to release it- assuming there is no extra hardware working onboard (which for the GBA there was pretty much was nothing unlike the SNES and to some extents NES although that only means you get to worry about the code issues) you then get to worry about those that push the hardware to the limit/use some of the quirks of the hardware. To emulate this properly you need more power which is why emulators like bsnes work so well and are so demanding unlike the likes of Zsnes or SNES9x.
Short version- Janthran you have kicked off/found yourself in a kind of "well in theory....." thread as opposed to those wishing to get things done/create something you can run as it would on the original hardware.
To this end you should probably note emulation (especially of custom embedded hardware on radically different hardware) is hard area of programming, programming for embedded devices is nicer but still requires a bit of thought and emulators written to run on said embedded devices is often the realm of a masochist and doubly so when pushing things as hard as a GBA emulator does. Short of the options mentioned already nobody is holding their breath for a gaming grade emulator in the near future or at any predictable point in the future. Nice tech demos, ones you can try to wrap around certain games and modify to work (game specific emulators- there was a nice one for international karate in the early days of the DS if you fancy reading up on the concept) and one you could use to pull apart a game (not that you would really want to when you can just do it on a PC) sure but not gaming grade.
Back to your most recent question theoretically yes (and it will also require unsigned code) but the key part of that sentence was the one that dealt with shortcuts in emulation. Ignoring the very real issue of hardware specifics (only working for a certain save type or something) which are not necessarily a simple hack to add (back) in for various reasons consoles can have many modes of operation and for a truly accurate emulator (which some games require) you get to emulate each option with the timings it had on the original hardware. Somewhat elegant solutions to problems can be achieved when you know the hardware; for instance if something has not completed on another core by a certain point you can tell the game to jump somewhere else but if your emulation of the other core runs at 50x the original speed under the assumption of "who cares" it will almost be assured to have been completed and the failure to complete a task will never happen when the game expects it and it will never jump where it is supposed to go and thus your emulation can be considered to have failed. Now not all games will use such techniques (some are very easy on the hardware but that is OK as you can make great games using simple methods) and these are the ones that work which is enough to release it- assuming there is no extra hardware working onboard (which for the GBA there was pretty much was nothing unlike the SNES and to some extents NES although that only means you get to worry about the code issues) you then get to worry about those that push the hardware to the limit/use some of the quirks of the hardware. To emulate this properly you need more power which is why emulators like bsnes work so well and are so demanding unlike the likes of Zsnes or SNES9x.
Help Roms To Heal 
