How to say no to bridesmaid hair styling because of my wig?

Have you tried just saying that you'd like to do your own hair on the day? Maybe try having a private convo with each bride and say that you're really not comfortable with the professional styling idea. I had a friend that was self-conscious about her ears because they stick out, so she hates undo's on her. The bride was ok with not requiring the undo.

That one bride is overstepping by requiring you to get your hair done in a particular style. Unless they're paying to get your hair done, they are both overstepping by making you pay for a professional hairstyle.

I don't think you're under any obligation to discuss using a wig with anyone. That's private information, and no-one's business. If it gets to that, I'd just probably attend the weddings as a guest.

Wow, well this is sounding more and more bridezilla by the minute. Honestly I'd probably just drop out politely. It's clear that you're being used as a prop for photos. You don't owe them anything if that's their expectation of you. Imagine making someone do something uncomfortable just for photos.

These women are total bridezillas to demand you wear your hair in a specific style and contrary to what some may believe to insist on professional hair and makeup, even if they are paying. These things are traditionally personal choice. You are an honored friend, not a prop. Anything beyond consulting on a dress and having some discretion there is inappropriate.

That said, I suppose you can try telling the bride that you require this accommodation and that it is for personal reasons. If she still says no, then that's not a friend and I would step down.