I think this question becomes more of, if the line judge is unsighted and there is a camera in place for that line, does it automatically get reviewed or is it still just considered a let.
Technically there is no "challenge". The challenge cannot be deemed "correct" or "incorrect". If a player was able to ask for a review of such shot, I'd assume they might as well review all unsighted lines that have a camera anyways.
^ To borrow from cricket. The fact that a decision cannot be made by the eyes on court should mean that the decision must automatically be referred to DRS by the umpire. This would make the Let obsolete.
The review is initiated by the umpire, so it does not originate from either players' side and therefore, is not counted out from their "review allowance."
Maybe BWF should clarify such a dispute in their Laws e.g. A challenge is not allowed in a Let call by the umpire.
If the Let call makes a difference in winning or losing a match, it matters a lot. If the LJ was distracted or not paying full attention and he signaled "unsighted" to the umpire who called "Let" this wont be fair to the player who claimed his shot was "in".
So can a Let call be challenged if a camera video is available for review?