A quick couple of update pictures. I got the building panels clad in DAS clay last night. Admittedly it took a few hours, the most fiddly bits getting it neatly round the windows and doors, so as not to obscure the matchstick frames.
Unfortunately my idea of the lino roller didn't work out. It's quite a stiff roller and you need to push down firmly to get it to roll. So it either gripped the clay and lifted it, sticking to roller, or, after I had put some lubricating cream on the roller, it just slid over the surface without rolling.
So I just resorted to firmly pressing it down with my fingers, having first teased the clay out into small pieces as thin as I could manage. Then just rubbed it down smooth with wet fingers, with a small bowl of water to the side as I worked. I used a small scultping tool to scrape the excess away from the window and door frames.
So the clay layer is probably only a few mm's thick, and adhered easily to the MDF surface. So long as you keep your fingers wet as you work it doesn't stick to you too much.
So this should be good and dry when I get in tonight to etch in the brick pattern.