I personally dont see the need to enforce what people can/cannot wear.
As we are all consenting adult, and are fully aware of what can happen by a 300fps BB hurling at us why enforce additional protection?
For example, at outddor games during the summer, I can often be seen running around topless with just my tactical vest on ( and just goggles as compared to a full face mask) knowing full well that I can, and likely will, take a 3+ round burst from a 400 fps AEG and am fine with that.
I have signed my waiver and am responsible for what happens to myself. As long as you are covered, by the waiver, I wouldnt start enforcing full face masks that most of us are trying to avoid by using tactical goggles when we can.
As you said, full auto fire will be rare as is. If a person cannot take a single 300 fps hit on bare skin, they are likely in the wrong sport.
Do you have any MP5 accessories that you're looking to get rid of just let me know.