A bit of an odd sounding idea I'm sure ... but I was wondering ... would it be possible to have a mannequin where through some means you can change it from male shape to female shape and viceversa depending what you wanted the clothing/armour to look like?

I think the answer on how it can be done will depend on whether you want to be able to change the six of the mannequin while it is wearing clothes/armor, or if it's okay to require removing all items from its inventory before the gender change. The actually swapping of an opposite six mannequin seems like it is pretty easy to do with vanilla scripting, but you'd be replacing one NPC object for another, and would lose inventory items unless you manually removed them before and replaced them after the change.

Perhaps a warning message would suffice? Would there be a way to detect that there are items on a Mannequin and give a message to the vain of "You have to remove all items from the Mannequin before you can change its gender" ? With something to stop the script activating if there are items? Just some ideas.

Most mannequin scripts for movable mannequins already do warn you when you go to "pick them up". Detecting whether they have something in inventory is more difficult with vanilla scripting (I always have to make that qualification because I don't know about the script extenders), so it's safest to just always ask, as the player may have put some weird item in the mannequin's inventory for storage.

But sure, if you warn before the six change, it should be a pretty simple modification to existing mannequin scripts to add the option to change gender.