neimod and yellows8 seem to be making some decent progress without decapping. I'm not actually sure if decapping would be extremely useful in this regard; I believe it would mainly give the bootloader etc., which probably doesn't have access to most hardware anyway. I'd guess the bootloader's main job is to load code from NAND (which won't be accessed by decapping), and then code from NAND would then have the code for accessing hardware.
(If you're familiar with the Wii, I'd imagine decapping would only really give boot0, which is basically a small stub to load boot1, which then loads boot2, which then loads IOS, which is what actually has access to all hardware. boot0 wouldn't contain the details required to access, say, video output, because it never needs to do that; IOS is loaded before any video output is shown - this information may be a bit inaccurate, I'm just guessing here, but I think it would be something like this).
On the other hand, decapping should probably give the keys for decrypting titles from the NAND, which could then be reverse engineered to figure out how to access various hardware.
Either way, it would probably be useful, but not necessarily essential/pivotal to 3DS homebrew.