Interesting topic! I would be more than happy if men thongs become socially accepted as well as "one of the popular men's underwear types".
Definitely the first step would be for thongs to become socially acceptable. Big brands need to take that initiative.
Any premium/luxury brand (Versace, Dolce Gabbana, et al) would first need to showcase mens thongs in major fashion shows.
Then, may be, Armani coming up with "Skimpy" underwear collection for men. You can't just market thongs. There has to be a gradual progression. So, Skimpy collection would focus on 'sexy and comfortable' designs. Educating how pouches and skimpy coverage bring best of both worlds (sexy and comfortable). Comfort should be the key in the marketing campaigns.
Digital/print ads could focus on male celebrity wearing low coverage bikini undie such as 3/4 back coverage with decent pouch.
That's how the Skimpy collection would look:
3/4 back coverage, 2/3 back, 1/2 back, and then Thong
These would be available with different pouch sizes and with thin or no waistband.
In a nutshell, a big brands focus on 'comfortable yet sexy' and celebrity endorsement could just make it happen.
Oh. I guess I am being carried away and getting too demanding.