"My dog Ginger (Rott/Mastiff rescue) is generally a very loving dog with my husband and I, however when visitors come, particularly men but especially men with facial hair, she flips out. Barks and is very nervous/fearful. We've tried having the new visitor give her treats and introduce calmly but more times than not she will bark incessantly. She has not bitten anyone but obviously we are afraid she will one day. We've had her since she was 8 weeks old and always had visitors without any issue till she was about 8-10 months old. There is no known trigger – no incident that would have "turned her". Any advice you can provide would be godsend!"
So far so good. You've observed Ginger's behaviors and narrowed down her fear to (1) visitors, (2) visitors who are men, and (3) visitors who are men who have beards. Excellent work. In training and behavior terms, those are Ginger's "triggers."
And since who dogs are is made up of both their experiences and their genetics, there is not always an event or experience that "turns" or changes dogs. Dogs continue to mature socially up until they're about 2 years old. During that time especially their personalities continue to change – just like people's do.
Now to get a bit of handle on this, I have a couple of ideas and YES, we'll need to go slow and be careful. We do not want to ever put Ginger into a position where she feels so overwhelmed she bites.
So first, we'll start with the lowest level stressor and work our way "up." Distance and timing are our keys to success here combined with LOTS of awesome treats from YOU (not from the visitor). That way, Ginger can be far enough away to be comfortable.
Eventually you'll be able to get closer, but not for a while. Remember only reward her when she's comfortable, i.e. not barking. If she's barking you're too close.
Now, I know this is odd…but I'm also thinking…how would your husband feel about growing a beard? If Ginger's good with him (although you say "generally"…which worries me a bit) it might be interesting for him to grow a beard – to help teach her bearded people can be nice too. Certainly dealing with significant levels of fear is hard on your family and your dog and could potentially – as you mentioned – become dangerous.
Consider contacting a qualified veterinary behaviorist for assistance. An excellent booklet called Cautious Canine by Patricia McConnell, Phd can help provide you with solid guidelines as well. You can order the booklet from Amazon. The most important thing here is to help Ginger and keep everyone safe.