I can not see the need of usefullness. Major department stores have cameras all over. I have seen where they can track a person before they enter the store through the windows and doors. As the person came into the store throught the store up the escalators where he tried to return some merchandice left the store and forced a woman to return and try again.
I could think of lots of reasons why it would be easy and a good way to go
- mannequins are everywhere - to hide in another object that object has to blend with the rest of the setup - a mannequin will always blend
- If the mannequin was on a small lightbox, nobody would think twice about a cord / switch running to a power cord
- If really doing, the camera would not need batteries etc, would be easy to manage.
- It would be easy to move the mannequin around, and you could even install a long life, large battery kit if need be - that could charge when it was plugged in.
On the whole, even if not commercially available, it would seem to be quite easy to build and maintain, so I would be betting that some have done it.
Ah…the things you learn at the Dope! 
I work in retail, and I don’t understand why a store wouldn’t want their security cameras in plain view to deter potential shoplifters.
I used to work in retail also, and in a casino.
I would suggest that because sometimes you want to deter, but you also recognise that no matter what you can’t deter everyone - so some you want to catch? And if you want to catch you keep it hidden.
If word got around that “shoplift from store x and you get caught” that is equally as well a deterrent right? And probably longer lasting than any visible cameras