Aside from looking awesome, body piercings make great attachment points for all kinds of bondage. Most, if not all, piercings can be utilized to enhance a ponyplay experience, but some piercings specifically can be of great benefit in controlling a pony.
When using piercings for attachment or for any purpose where force will be applied, keep in mind that thinner jewelry may rip through the skin. Thus, make sure you use an appropriate gauge for the use you intend.
Nipple piercings are probably one of the more common "functional" ponyplay piercings. Reins can be attached to a ponygirl's nipple piercings for that extra bit of control. Simply atttaching reins to the nipple rings and using these nipple reins is usually enough to control a pony. Plus, when you use nipple reins, you can use a ball gag instead of a bit when driving a pony.
However, the best and most fun (in my opinion) way to make use of nipple piercings via the reins is to attach short reins from the nipple piercings to the ponygirl's bit. Then attach a separate set of reins to the bit. Now, when you pull the pongirl's reins, the bit will be pulled back into her mouth, which will then pull on the nipple rings through the second set of reins attaching the bit to her nipples. The dual action of the reins (via pressure on both the bit and nipple rings) is hard for even the most recalcitrant ponies to resist. You can add bells to nipple piercings as well for a nice jingling when the reins are pulled.
An alternative to the above setup is to use a single set of reins from the nipple piercings through the bit rings to the trainer's hand. This arrangement is similar to a running martingale except with an attachment to the nipples instead of the girth/waist harness. Of course, you could even run an attachment from the girth/waist harness through the nipple rings to the bit. Kind of a super(-effective) running martingale (especially if the martingale pulls on a crotch rope on the waist belt).
Tongue piercings can be extremely useful for control of a pony's mouth. Bits with mouthpieces that extend into the mouth can be (or be made able to be) attached to a tongue piercing directly. This arrangement is quite effective in controlling the tongue.
With a tongue piercing attached to the tongue/bit port (i.e. mouthpiece that has protrudes into the mouth) portion of the moupiece of a Pelham bit, the pony's tongue is forced down in the mouth preventing the pony from playing with the bit (though this can cause significant drooling) even when there is no rein pressure. If the snaffle reins are used, there is not much effect on the pony's tongue (maybe pulled back a little in the pony's mouth). If the curb reins are used the pony's tongue if forced up to the soft palate along with the tongue port (this can produce a gagging effect depending on the amount of rein pressure and length of the bit shanks). Finally, you can use a third set of reins attached to the cheek rings of the bit. This will produce the opposite effect of the curb reins: the pony's tongue is forced even further down and out of the mouth, which can be quite painful.
Also, tongue piercings can be used to attach tongue ties, which immobilize a pony's tongue by holding it out and down.
Septum piercings can be used as an attachment point for a lead rope. This provides an effective way to lead a pony without having to use a halter.
Genital piercings can be used as atachment points for reins as well, but care should be taken here not to put too much pressure on these piercings.
Corset piercings look really cool, but they can be functional as well. If they are thick enough and there are enough of them, they can be used to a pull a light (and riderless) cart. However, I included them here simply because I think, if they are well done, they look absolutely beautiful.