One client who submitted an order form stated that they would e-mail me their reference images after securing the slot, which is perfectly fine. When I contacted this client afterwards, he told me that he is a trans man and was looking to get a waist-up illustration of himself after his eventual top surgery. That is something I 100% support and would be happy to do for him.
However, when I asked for reference images, he stated that of the few selfies he felt comfortable sharing was of himself topless - again, he has yet to have any surgical procedures done. This makes sense to me as reference material, since it would give me a better idea of his body type, proportions and any scars or tattoos, but I'm pretty certain it would still legally be considered NSFW/18+ material.
As it stands, I think I'm going to tell him that if he can provide proof that he is above the age of 18 (a scan of his driver's license or state ID card with other private information censored, or similar), a nude photo would be no issue for me.
I'd like to get the input of other artists, however - for the past 5 years or so I've been strictly a clean artist; I never draw pornography but I have drawn tasteful nudes, both for clients and for myself. But since I do almost entirely draw clothed characters, the vast majority of my reference material from clients are also fully-clothed. I feel a little out of my element here.