It is said that J.K. Rowling was planning on making Dean Thomas and Seamus Finnigan a couple, but she ended up not doing it because she didn’t want to take too much attention away from the Trio. Therefor technically, yes, Deamus could be considered canon.
The Twilight Zone - Originial Series (1969 - 1964)
There is a fifth dimension beyond that which is known to man. It is a dimension as vast as space and as timeless as infinity. It is the middle ground between light and shadow, between science and superstition, and it lies between the pit of man’s fears and the summit of his knowledge. This is the dimension of imagination. It is an area which we call “The Twilight Zone”.