I have tried the Matchbox keyboard and it is working reasonably well.
I used the same construct as above, with a USB connection between the screen and the RasPi. I have been using an external battery pack (2A) and various USB power supplies. In all cases I got the power indicator/rainbow square, but the system was running well.
Using a plastic tablet stand from IKEA (ISBERGET, http://www.ikea.com/de/de/catalog/products/20302596/
), I have built a frame for the screen with the RasPi attached externally to the stand. I will try to post some pictures of it tomorrow.
In general the Pi and the screen were working well, but especially in combination with audio control, the system was hanging several times.