A leather wrap is probably your best solution as Fitx suggested. Leather will often swell up when wet so leave plenty of extra space. I once had a mast with a leather wrap to protect the area where it passed through the seat support. This made it very difficult to remove the mast following a wet day of sailing so I ended up removing the leather.
My leg-o-mutton sail rigs have a similar problem which has been resolved by wrapping some old cotton line around the hardware which was causing the wear. I have never seen this type of wear advance to the point that it causes a structural problem so you could also just ignore it and simply revarnish the mast more frequently when it starts to look bad.
On big old sailing vessels they used leather (often even varnished rawhide on some things like mast hoops) But leather by itself is kind of sticky, so they would grease the areas with tallow. You probably don't want a bunch of rancid tallow on your canoe and most other greases tend to eventually get all over other stuff. I've seen small boat spars with areas on their masts wrapped with sheet brass and also fiberglass which works, but isn't very pretty. One of the other common methods on traditional boats was to attach a series of short, vertical wooden "wear strips" to the mast in those areas. These would be small, raised ridges made of some sort of really hard wood and spaced slightly apart around the mast (or sometimes only on the aft side of it). The boom or gaff fitting bearing on the mast then rides against the top surface of the ridges and doesn't contact the softer wood of the mast itself. If done right, it's a pretty elegant solution. On a canoe-sized mast, you're probably looking at ridges maybe 4"-6" long, 1/4" or so wide and 1/8"-3/16" tall, spaced 1/2"-3/4" apart. They would look something like this drawing. If I was doing one, I'd probably glue them to the mast, varnish over them and then maybe sand the varnish off of the outer surface of the strips and replace it with oil finish to give something easy to maintain. You might also be able to do the same sort of thing with small strips of sheet brass or even stem-band brass.