Here's an idea.
Find a photo-sharing platform that allows for nudity, and mix a few 'accidental' photos into your regular social sharing.
Take Flickr for example, in most cases if the picture is labeled correctly (marked as adult, not visible in public searches) then they'll allow it.
Downside is they won't see the picture without an account (I think) on Flickr, so there's no guarantee they'll see it. But, that totally adds to your plausible deniability.
If you want to take the non-photo route, but do it in person:
This idea is a little more daring, if you and your friends meet in person: have her reveal parts of her body by accident. Simply being a little uncareful in a short dress is enough (bonus point for going without panties); they'll notice. A low cut top does wonders too, bonus points for going braless.
Another idea if your friends are over for drinks. Your wife goes to bed early (about 15 minutes before the others leave), and she leaves the curtains open and the lights on while undressing and undoing her makeup, etc..
You turn off the lights in the living room and watch outside as they leave if they're looking.
Keep in mind, she'll slowly be known as a little bit of a sl*t, with respect. Your friends will start to expect to see her every now and then, this opens the door for Far more casual exhibitionism. (walking in on you and your friends, coming just out of the shower, or sitting legs spread with the rest showing everything).
--Happy to share my thoughts, good luck and Have fun
We're only human after all,