Somebody expressed concern to me off list about Boric acid being a skin and eye irritant. So the internet tells us, but then again so is table salt and shampoo. Anyway, empiricism is the only way to really know, and I happen to have some on my shelf. About an hour ago I put a saturated solution of boric acid on a paper towel and put it on my left arm to dry while I work. It's dry now and my arm feels a bit crusty, but otherwise fine, comparable to a day at the beach.
I also have used it on a jumpsuit with only my skivvies on underneath. My sweat probably dissolved a lot of the boric acid onto my skin and since it was Burning man I didn't shower before bed. I did notice my skin was a bit itchy the next morning, but it could have been all of the methanol and numerous other metal salts I was using for colored flame effects, or the many burns from setting my skin on fire, or the powdered magnesium I was throwing at people's fire art with my bare hands, or the other kind of acid.(I am not a role model) After brunch, I took a spraybottle shower and felt fine again.
Boric acid forms a salty coating over your fabric, and it has a low PH. It might itch in your more tender areas after a while. Again, this something you would put on the outer layer of a costume, not a g string.
I'll let you know if this crusty patch gets itchy and put the results of this experiment on the Sparkworks blog.
Ben