I've always loved this idea. There are many more web resources, and it is not necessarily a hard skill to learn. There are a few examples
online of beginners making successful ceilings.
We tried this a few years back, but we had a very hard time finding quality tiles at a reasonable price. On top of that, you really must work during dry spells, as the plaster of paris mortar is really very susceptible to moisture. If it gets wet, the whole roof can collapse.
My advice is to build something small, like less than 8 ft by 8ft, just to get a feel for the technique. You will nee to do a minimum of 2-3 tile layers, and then a
water proof layer.
Here's a beginner one done as a test:
http://www.umanitoba.ca/cast_building/research/masonry/catalan_vaults.html