Yes, I have.
I have made a few long fillet knives- 15.5". They are a tad more difficult to keep a steady taper than a standard fillet knife. I prefer to flat grind them with the handle in place. You need to be very careful that the belt does not touch the handle front. But, with proper tracking adjustment and patience when switching directions, you can do it. The photo shows the 15.5' next to a standard length which can be anywhere from 6-7.5".
Milt