Hmmmm… it might cause some headache, but sure you can. It would be a good idea to add a stich to each end for seaming insurance. I guess you would just cast on the amount of stiches and follow the pattern like regular… that is quite a smart idea, way to go
Fair Isle is easier to knit in the round, since you’re always looking at the front of the work, but it can be done flat. Just make sure you read the purl rows from left to right.