it would be very expensive. No one is going to do just one crank or even 50, the process is very extensive. I did a little research on it a while back, they rotate the part (or in your case the crank) on three axis, it also needs to be 5k degrees, and it needs to be in a vaccum. Places do it, but not just for one crank. Maybe 100 or more then it would be about 15 - 20$ a piece.

Didn't look into tri nitride coatings, which are half as durable.

You don't need it anyway. the steel on this crank is very hard

would be cool if you could just by the coating and just dip and go