I was sitting in 21H on an AMS-NRT 744 not too long ago and overheard a man talking to a senior FA/purser in the galley about doing just that.

I believe he was deterred by the price, which IIRC was in the mid $1K range. It may have been based on actual fare paid (as opposed to the difference between C and S/Y), but I remember thinking at the time that FD miles seemed quite valuable