PSX documentation: http://www.problemkaputt.de/psx-spx.htm
I wasn't going to start this rant, but whatever...
N64 has several LLE plugins for the common emulators, plus MAME and Cen64. There are varying degrees of support for features; for example, only Cen64 implements the cache. 64DD has been implemented in several emulators over the last couple years as well at a somewhat high level, good enough to run retails disks. Hardware needs to be developed to read disks at a lower level, and the microsequencer needs to be decapped/reversed.
That said, state of emulation is no impediment to hacking N64 titles--and I'm saying that as poster-child for managing to hit every possible edge-case there is. The idea this is an issue is utter nonsense.
Honestly, you don't even need to understand half the hardware. It helps, sure, but for the most part you won't need to touch any of that yourself. What is important is understanding threading and messaging. For instance, a common problem is when people seem to think they can just jump out of a function and write in their own DMA. They don't realize that there's a thread that manages E/PI requests and it uses messaging to ensure that the device is not utilized until it's ready, delaying requests until they can be filled. When you do things behind it's back you could sporadically execute a DMA when the device isn't ready, and the results range from bugginess to full-on crash. Not fun to debug. The alternative is to figure out which message queue takes requests and send it one. It's foolproof and much lazier.
Number one issue preventing most hacking attempts are compression-related, number two is shoving files back in, number three is that the sheer volume of data you need to deal with can be overwhelming, and number four is time.
N64 hackers don't really come here, and neither do people come here for N64 patches. GE:X is hosted across several sites; rhdn's download count is 2% that of GEvault. Contrasting the stats at mediafire and the counts for my patches here, rhdn averages out to 5% total downloads. In fact, most of my work either isn't or can't be submitted here for one reason or another. For the most part, N64 hackers congregate on IRC, on boards dedicated to particular titles, and collaborations through Github.