If working at petco counts, I don't see why part time farrier work wouldn't.
Have you been been certified by the AFA, or are you in the process of getting certified? I personally know very little about farriery, so my knowledge only comes from having talked to just 2 well established farriers, but both spoke very passionately about how these 12 week programs are just a starting point in a farrier's training, and that they do not produce fully fledged farriers. Both had strong opinions that regardless of whether someone chooses to be certified or not, a bulk of the training occurs in the field with a mentor after farrier school. They didn't have very good things to say about people who operated independently after their 3 months of schooling. Obviously, as both had gone through long apprenticeships and had worked pretty hard it sounds like to pass the certification exams, they may very well be biased. Again, I personally don't know enough to form a judgment on the matter... but adcoms might (and possibly agree), so I'd be very careful about how you word your experience.
That being said, I think this experience will not only set you apart, but also set you up for a compelling PS. You have a huge advantage in that you have seen first hand the importance of hoof care, and I'm sure you can eloquently articulate its relationship to medicine. You also have the advantage of knowing and possibly gaining trust of clientele that overlaps as veterinary clientele in equine medicine. I think it would work hugely in your favor to talk about this niche... since we all know how peculiar these horse people are, and not everyone can easily work with them (I certainly can't).
