Atletico did it (younger) with De Gea, so too Parma with Buffon. Depends how good he is really and who's ahead of him but it can happen.
Sure, Iker too at Real. Casillas & Buffon are one in a million talents, both in the best ever goalkeepers group. They were young keepers that their clubs believed in and made that investment with the future in their minds too.
Just don’t see a club down the leagues or abroad taking that chance with a loan keeper. Could happen of course.