Indeed it is already possible using a non-uniform cartesian transformation within a mapped item, but I wouldn’t exactly call it “nice” 
With this method, perhaps some of the object is mirrored, and others not. This adds to the confusion about the implied constraints of how mapped representations should be used.
Further reading on this topic:
- Controversial proposal to drastically simpify mapped representations
- Is IfcProductDefinitionShape a 1:1 relationship? - #4 by angel.velez
On this topic I’m a proponent of “less is more” and would want to rethink how mirroring (and other common geometric modifiers, like arrays) can be applied without overcomplicated the already complex representation tree.