You cannot currently unfortunately.
The problem is really just an API one. Since currently you can only ever
query a full collection, you cannot apply writeTime() to only an element,
and applying it to the whole collection doesn't make sense, in the sense
that each element have a write time as you said.
We'll likely allow to query individual elements of collections in the
future, at which point allowing to get the write time of said individual
will work. But let's say that today we just don't have a syntax yet to make