Acrylic paint dries waterproof so you shouldn't have any problems, though I'd still be careful. Just allow it enough time so that the paintwork is completely dry, let it sit for a day or preferably two. If you use sealant, make sure that is waterproof too! (Mod Podge isn't because it's glue-based.)
For straw curls and such, I dip the hair into boiling water holding the pony by its feet. Then put a towel between the body and hair while it dries. Good luck!