Some thoughts on the topic: admittedly, probably not very useful.
gcast1 itself operationally makes sense regardless of the kind of the argument you're skipping past.
gcast1 :: forall c t t' a. (Typeable t, Typeable t') => c (t a) -> Maybe (c (t' a))
This is an operation we can define pretty easily, however we want. It works more or less by definition. It was included in the original paper.
2)
dataCast1 on the other hand is an member of the class, and it needs that 'Data d' constraint or it can't do its job.
dataCast1 :: (Data a, Typeable t) => (forall d. Data d => c (t d)) -> Maybe (c a)
Since then we obviously picked up polymorphic kinds, which muddled the story about when a data instance is for a type that looks like `T d`.
On the other hand, the user will be the one calling this method, and they'll have to take Data instances for d and use them to generate c (t d). Knowledge of if argument is of kind * is purely local, though. If t is Typeable and d is Data, then (t d), d is kind *, t :: * -> *. So to call dataCast1, the user has to already know the argument is kind *, and that the type t that they are trying to use (maybe not the T in the data instance itself) is of kind * -> *.
There is no contradiction in trying to call this on a data type with the wrong kind for it to succeed. T is not necessarily t. It is our job to check if it is.
Supplying the default shouldn't lock our data instance to the form T a. If for some reason adding this default would break the instance We can make a more interesting default that does something like look at the kind of the argument first to determine if it is kind * before proceeding after casting to ensure the kinds match.
tl;dr yes, it would seem it would be worth fixing the instances of Data produced to supply dataCast1 when the kind of the argument is polymorphic. Otherwise turning on PolyKinds in a package will simply break dataCast1 for basically all of its data types.
-Edward
P.S. Ultimately, in a perfect world we'd be able to unify dataCast1 and dataCast2 with some tricky base case for kind k1 = * and induction over k -> k1. Off hand, I don't see how to do it. I played for a while with trying to write a higher kinded Data to support this, but my scribbles didn't cohere into usable code. gfoldl pretty much locks you into kind *. I even tried playing with profunctors and powers here to no real avail. I do have some old Data1 code in an syb-extras package that I use to write a few 'impossible' Data instances, but that seems to be solving a different, if related, problem, and doesn't scale up to arbitrary kinds. Ideally there'd be a plausible Data that makes sense for other kinds k, and everything could become dataCast1, w/ dataCast2, and the missing higher versions just an iterated application of it. I just don't see that there is a way to turn it into code.