Hi Dave.
I just saw your email. The closest functionality that exists right now is the "flip X/Y" options on a layer's properties. This will mirror along the X and/or Y axes.
There are plans to add ability to rectangle/lasso select and independently manipulate specific areas/sections of raster-layers in the future. Right now the user guide is being updated for v1.44 which will be released on January 1st if there aren't any serious hiccups. :)
Charlie