Does the Nikon D90 have custom firmware? Doing a quick Google search didn't really find anything. And, along similar lines, is the D90 even capable of flashing its firmware? Or are we basically stuck with the stock firmware the camera comes with?
There is no way to change the firmware of Nikon's cameras other than upgrades from Nikon.
For some Canon digital compacts, there's an unsupported project called CHDK that can add certain features to the cameras.
Allowing Random J Hacker to load weird firmware into his expensive camera is a customer support nightmare. For every person who knows what he is doing, there are 10 that will brick their camera, clog support channels, and destroy what already slim margins exist in the camera business.