JSFL - Detecting When Element Has Been Flipped

I'm writing an exporter in JSFL, to export Flash animations into a format that can be replayed in a custom player. The exporter basically iterates through the timeline and through all the elements at each keyframe, and writes out the element's name, position, rotation, scale and a local offset. These are read into the custom player which feeds the data to a sprite engine to recreate each frame of the animation. What I want to be able to do is detect whether a given Element has been flipped (i.e. in Flash you Select the element (a symbol), then Modify->Transform->Flip Horizontal) so that the exporter can include that information too, allowing the sprite engine in the player to flip the UVs of the texture to replicate what's happening in Flash.

This would be useful for (say) using one symbol for a character's right hand, and just flipping it to be their left hand, rather than having to create a whole new symbol. Unfortunately I can't see any way of finding this information out. None of the information I have available for the Elements seems to imply that any kind of flipping has occurred. How can I detect flipping? If it can't be done algorithmically, I'd settle for the animator having to manually indicate that a symbol had been flipped (by creating some kind of plugin that gives them a tick-box which writes a value into the Element with setPersistentData(), for example), but I don't know how to make that sort of plugin either.