Here's a company offering to make paintable moldings out of a flexible plastic that can be bent:
Here's a company in Australia that makes pre-notched MDF board for bending under the trade name "Craftform":
The Laminex Group
What scares me is that MDF is pretty weak stuff to begin with. If you then notch it the way these Aussies are doing, it'd be a trick to even pick it up without breaking it.