In the UK, aside from the risk of falling off, the structure gauge of the line probably would not allow space for the sidesaddle driver. We definitely could not do it here, in Denmark where I now live, as the clearances between platform signs and the cab are close to zero. On some of the engines, there are small warning signs reminding engineers to duck back into the cab.
In SA, worst case the driver falls off, and the fireman has to stop the train. Injuries to the driver are not taken seriously.
The Sandberg family used to drive Minnesota Transportation Museum #328 sitting in the window ledge, back to the outside. I tried riding like that a few times when employed at the Strasburg RR, but I was gently told to stop. At speeds of under 30 mph, it is not bad, but there is that risk that you loose attention and roll out backwards onto the right of way! It does really improve your line of sight ahead.