Part of the reason it's so hard to get code into NSS is because no one really knows who owns those bits, and so they linger in obscurity. I think my TLS Channel Bindings patch is lingering on 3 or 4 years now.
However, I think you're doing a disservice if you only look at how easy it is to add code. As important is how easy it is to remove code. With NSS, it's far, far harder to remove code - and that makes it far, far harder to reason about any new code you might want to add, for who knows what grues are lurking in those shadowy depths.