mmm, nintendo only released the microcode for RCP programming towards the end of the N64's shelf life, and with minimal support/documentation.
As a result, only a few games ever really made use of it, and meaning the majority play fine with HLE emulation, especially when combined with a few hacks for where they don't.
The issue is not so much the ucodes, but the fact that they are custom. They do undefined things, perhaps different for different games. Depends on what the developer used it for. To emulate the reprogramming, one needs to have a major restructure the way that current plugins work.
Maximum effort with minimum results.
Most people are more focused in concentrating efforts elsewhere, improving performance or current compatibilty issues.
http://www.emutalk.net/showthread.php?t=40640
Take a look here for the z64 plugin. This may give you good results, maybe not.