Would a keyboard cover prolong the life of these rMBP keyboards?
my understanding is the culprit is dust getting underneath the keys and screwing up the sensor,
First, though I don't really know, I very strongly doubt that a little dust is screwing up these keyboards.
Second, take a look at these machines as they close. There are bare microns of clearance. There's no keyboard cover that would work (even if you could tolerate typing through it) that you could permanently have in place. I guess you could carry a keyboard condom around with you separately and put it on when you were ready to type...