So I'm working on a few at the moment.
1. The girl I mentioned above who I'm meeting up with in March. I was messaging her about her and my wife getting her tits out in ibiza before me and her hubby starting going out with our respective partners and what a shame we missed out. I got a laughing emoji back.
2. Another woman who I know isn't shy and still gets them out on holiday. I mentioned that I would love to be on a beach with my wife with her "sans top". I got a laughing emoji from her too.
3. The little Indian girl I mentioned. She went topless with my wife when me and missus were just starting out and they went on a girls holiday to ibiza and all four of them were norks out for two weeks. I was left at home unfortunately. I have tip toed round the subject with the Indian girl but I've talked about the beach and I know how free they were. Her response tells me she knows what I'm talking about. I tried to tee up a holiday with her and her husband with me and missus this summer but it seems to not be happening - I was really hoping I could get the girls to show their tits if we did.
4. A black lady we are friends with who used to get them out back in the day. I have no idea if she does still, but I've managed to let her know my wife is used to taking her top off at the beach. I said "Oh tell me about it. What happened to lying around for a couple of week with a crate of beer, a glass of rose, a good book and wife sunbathing sans top. I miss those days, hopeful they will make a comeback " that got a heart emoji!
5. Not a woman, but a male friend. We went on holiday as two families a few years back. Both wives kept this boobs under wraps. I found some photos of the beach and messaged him the photo with " The beach/lake. Can't believe our wives didn't treat us topless sunbathing like all the French ladies there were. My missus would normally; sorry

" this got a laughing emoji so I followed up with "Think missus floated the subject one evening from memory but yours put her off the idea as she'd never done it before and mine wasn't willing to go it alone. You missed out there mate lol.

" and "Sorry, that was just me explaining the context of why she disappointingly kept her top on, much to our loss." which got another laughing emoji.
Sorry I know this is all very tame, but as I have never experience my wife with her tits out in front of my friends, nor my female friends having them out for me to enjoy, all I have is getting them to think, message, and react to the thought or memory of it. The fact I made my mate think about my wifes tits, and that I have had a chat with female friends discussing them topless and therefore their tits, goes some of the way. Albeit I would much prefer it if they messaged back with a picture of them, but that seems unlikely.