To summarize, the other big issue besides the leds and rev lights is the very limited button functionality on Xbox. Fanatec wheels only support buttons which appear on a standard Xbox controller (ABXY, LB/RB, etc). Any additional inputs such as toggle switches or rotary dials either don't work at all on Xbox, or they are duplicate inputs and can not be reassigned to other functions.
Bottom line is, things like rev lights and led panels CAN be done on xbox compatible wheels right now - Thrustmaster has their latest Ferrari F1 replica wheel to prove it. They also have their range of flight sticks and yokes to show that many more inputs than that of a standard controller can be achieved.
However, for whatever reason, Fanatec have chosen not to use whatever development method TM have employed, and do not currently offer any Xbox compatible wheels with rev lights, working led displays, or expanded non-standard button support. Some have said that companies like TM have used "loopholes" to make their products work, suggesting that such a method is inferior to how Fanatec would prefer to implement such things. For such features to be implemented "properly", it's been said that the Xbox OS /SDK needs to be "opened up".