A lot of EASA bashing going on with no knowledge to back it up. To get approval for any type or class rating and aircraft in it just needs a course and proposal putting together. I have approvals to train for some quite odd aircraft. As Head of Training, I write a course submit it and gain approval and have gained many unusual approvals this way.
What you have to do is demonstrate that you can meet the required standard and maintain it both in terms of the initial course and ongoing currency. When encountering a new type you have to show a path of enough experience or training on similar aircraft in order to Get an approval. As an Examiner I can then test.
The Do24 is a good example in point, I have many thousand hours in the Do28 ass well as being a seaplane examiner and could put a proposal together to gain appprovsl for training and testing on it. It's already been done on the same basis in Germany.
With a little determination annything is possible.