Guys. Think literally. Catching anything on fire would require the use of particle effects, which not only lag like a bitch but would be a bitch to code any kind of flame "spreading" or anything "catching on fire".
I suppose the closest thing that could be done is putting in a flame effect on every flammable scenery object in the map, then manually disabling them until hit by an arrow. I think projectiles in this game act as entities, so it might be codeable. Not practical though.
The most realistic thing would be replacing the standard arrowhead with an arrowhead "covered in flame" both strung and in motion, but doing blunt damage with a severe bonus to shields (IE: Better versus barricades and shields)