I agree with what you said, except for the case of where the writers would not have wanted to spoil him.
If that were the case, I don't think that was a smart move. Now I'm not a writer in any degree, but I feel the writers may have been able to give some kind of background on him if they were to introduce him then.
Now, there's a problem with what I just said. The episode was more centered on Rarity, so adding a new character that had relations to Twilight may not have coped well, as then the episode would have to shift focus to the siblings.
But this whole thing doesn't really matter, I'm positive he wasn't thought up until at least after this episode. Nothing they could've done.
(Goodness, I spend half this post debunking my own theory.)