There was also DSerial that might be worth a quick look
http://natrium42.com/
Back on topic speed should not be a problem as the GPIO which is not a speedy bus should still be roughly available in DS mode (it was how the guitar grip and rumble works- http://nocash.emubase.de/gbatek.htm#gbacartioportgpio and http://nocash.emubase.de/gbatek.htm#dscartridgegbaslot covering the necessary info). You might also want to look at the cheats/button hacks for the guitar grip as they will give a clue as to how things work (I believe I even had a look at them when some people were wanting to remap them beyond the ones available).
Frankly though you are probably better off forgoing all electrical logic and hardwiring things similar to say http://ladyada.net/make/usbgamepad/ if for not other reason than I would be quite surprised to see control hacking hit anything resembling automated (it is not terribly demanding as far as hacks go but still takes a bit of time). There were already a few things like turbo buttons and such made for the DS operating on a similar principle.
http://natrium42.com/
Back on topic speed should not be a problem as the GPIO which is not a speedy bus should still be roughly available in DS mode (it was how the guitar grip and rumble works- http://nocash.emubase.de/gbatek.htm#gbacartioportgpio and http://nocash.emubase.de/gbatek.htm#dscartridgegbaslot covering the necessary info). You might also want to look at the cheats/button hacks for the guitar grip as they will give a clue as to how things work (I believe I even had a look at them when some people were wanting to remap them beyond the ones available).
Frankly though you are probably better off forgoing all electrical logic and hardwiring things similar to say http://ladyada.net/make/usbgamepad/ if for not other reason than I would be quite surprised to see control hacking hit anything resembling automated (it is not terribly demanding as far as hacks go but still takes a bit of time). There were already a few things like turbo buttons and such made for the DS operating on a similar principle.
