I agree with xnine but would add that I have a bed where the head end can be raised as well as the leg-end. I find this is a position that suits me very well as it makes it almost impossible to roll over onto my side, let alone onto my stomach.
However, My second thought on this matter involves questioning whether it is absolutely necessary to be thinking that someone, who has spent ther lives sleeping on their stomach, should now be forced to do anything different. Surely, it is not beyond the ingenuity of mankind to devise a sleeping arangement to accommodate the sleeping in a downwards position?
My thoughts move towards the the massage-table approach, where the bed is adapted to make the downwards sleeper as comfortanble as possible by making a custom-made hole for the nose and face to sit in the right position to be able to breath. I would also look to have indentations so that the breasts are not unduly uncomfortable being squashed for long periods. Then there is the matter of catering for a stoma. This is something I have thought about before because I irrigate and it occured to me, that having a bed with a hole in it would be ideal for the stoma to drain off without all that annoying coagulation around the actual stoma site.
Working down the body, there would be no reason not to have an appropriate indentation to accomodate the feet and make the whole experience a comfortable one. I believe that a memory-foam mattress would top it off nicely, making an ideal and comfortable support for such a bed.
Given how expensive special beds (like my one) are, I would think that an appropriate bed to suit a downwards sleeper could be designed and built for less money than one that is opposite to what the sleeper really wants and needs.
In the UK, there are organisations with volunteer engineers and tradesmen, who take on such projects to help people with unusual problems such as this.
I hope these comments help in some way.