I had another unrelated keyboard problem that gave me a tiny bit of information.
(QPI would not connect to keyboard on start and restart, Apple updated the SMC to fix this problem but only on the new Ash Can Mac Pros, older Mac Pros are simply out of luck)
In this process I was shipped a replacement keyboard for free as a troubleshooting test.
I was sent the wrong keyboard as luck would have it.
In this case the wrong keyboard was the "A" model and my Mac Pro uses the "B" model.
The tiny bit of information I received is that Apple defines the "A" model as "Keyboard, Extended, Wired (2007)" and the "B" model as "Keyboard, Extended, Wired (2011)".
This can be helpful.
You can contact Apple and request the 2007 model as a replacement part.... OR.... use the serial number from an older Mac and request a replacement keyboard.
This will allow you to purchase a spare "A" model. ($50-$60)
(it also helps to know the model number of the keyboard but not much because Apple does not seem to have that as a reference)
How long the 2007 model will be available requires a crystal ball but we can bet that it won't be around forever.
The writing is on the wall.
DID YOU KNOW?
You can no longer purchase a compatible Apple Monitor for ANY Mac Pro previous to 2013.
Since the new model keyboard is incompatible (or nearly) with older machines and there is no Apple monitor compatible with older machines, the writing on the wall says.... you MUST purchase a new Mac if ANY peripheral fails.
(management calls this.... "doing good business")
To answer your question.
If a peripheral fails and your computer is still fully functional, you are out of luck IF.... you did not buy compatible spares when they were available.
I certainly did. (I bought both original keyboard and aluminum "A" model spares as well as three 27" DisplayPort monitors.... Displayport monitors work with Thunderbolt machines - Thunderbolt monitors do Not work with DisplayPort machines.... its One-Way Street)