The undertile systems such as onduline work, though produce a deeper roof.
However, from the tile manufacturer's point of view, the problem is not just one of keeping water out.
They have to consider the longevity of the tile itself. If the pitch is too low, the tile does not drain quite as easily as intended, and tends to remain damper for longer. This can be a problem in winter when frost comes.
I recently had occasion to do a roof with a pitch 3 deg lower than the recommended pitch, and the manufacturer refused to guarantee it. One suspects they may be overcautious, but a client may be prepared to take the risk that the tile may only last 40 years instead of 50.
You are correct about the tiles taking longer to dry on a shallower pitched roof, however, the onduline undertile system when installed with eave ventilation actually allows airflow on the underside of the tiles as well to allow them to dry out faster.
