Horse owner here. Dressage and cantankerous ponies for my daughter. Would we have been surprised if a farrier had been female? Maybe initially, but if she came highly recommended then no big deal.
We have only used farriers whom we have heard of through dressage friends because of the quality of their work. But, I'll tell ya this...if you're consistently available and good at your work, you are better than 90% of the farriers we have worked with. Reliability is key and as you know farriers are not the most organized or tactful bunch...use that to your advantage and blow the other ones out of the water.
Related to putting your gender in your ad, highlighting it calls attention to something that shouldn't matter. Consider that yes, some folks may not call you because of that and you may avoid those situations. However, there's no guarantee that even by putting that in your ad you won't have to face that judgement anyway since people really don't read things closely. I'm not sure there is any benefit to your growing your business and in fact, people who otherwise may call and you may develop a good working relationship may not call because of the hang ups which, through meeting you and your work, you can remove?
I think you do yourself a disservice in the long run, but that is easy for me to say that because I'm not in your shoes...it might really deeply bother you.
Part of me thinks the dressage world may be different than the western horse world, but maybe not. I suppose it could be entirely opposite, but many of the the dressage women my daughter knows would probably prefer it. Not sure if focusing on that section of horsemanship is even possible.
Mind if I ask what certification you have? Any therapeutic work yet? It's purely out of self interest (:D) because if you happen to be anywhere near Texas we are always willing to find a backup!
TLDR: don't put it in your ad, farriers rule and we need more of them!