Even if there have been customized 3DS consoles for 3rd party games in the past, we're still talking about games like Final Fantasy, Kingdom Hearts and such. Castlevania is sadly enough not even half as popular as those franchises. But yeah, I guess it could happen. Especially since the first Lords of Shadow performed reasonably well, so maybe expectations for Mirror of Fate are a little bit higher than usual in this series recent history.
Still, there is a problem in making a sequel to a PS3/XB360 game and releasing it on a Nintendo handheld. It's an entirely new target audience. Most of us in here play on multiple different consoles and handhelds, regardless of the brand, but many of the people who'll buy Mirror of Fate may never even have played the first game. So I'm not sure how "strong" this game really is... Though Castlevania was reasonably popular on the DS, of course.