Oral Sex
While tongue piercings and other oral modifications may not mean that a person is specifically looking for oral sex, most people agree that body modification can be advantageous to the oral pleasure experience.
Note that tongue piercings greatly increase your chances of passing STDs. Tongue piercings are especially susceptible to genital warts, so use protection.
Oral Piercings
Tongue piercings (and lip piercings to a lesser extent) can enhance the oral sex experience for the receiving party. Piercing mythology holds that if you primarily go down on women you should get a tongue piercing farther forward, and slightly farther back if you're performing on men, to maximize contact and control. Some may go crazy for the piercing, and others may not notice it at all. Silicone ticklers are available for extra stimulation.
Female Genital Piercings
Labial piercings and Christina piercings will make very little difference one way or the other for most people. The piercing provides additional stimulation.
Male Genital Piercings
Depending on the piercing and jewelry worn, you may want to temporarily remove it to enjoy more care-free oral sex. It's easy to chip teeth if the jewelry is large gauge metal.
Tongue Splitting
For most people tongue splitting doesn't make a major difference functionally unless the individual is already quite talented.
Meatotomy, Genital Splitting, and Subincicision
Some men find that the freshly exposed tissue is much more sensitive than normal genital tissue, whereas other men find less (or non-erotic) stimulation in those areas. Don't assume that a person's mods dictate what type of play they want — ask them if you aren't picking up their signals.
Genital beading won't make a significant difference in oral sex technique assuming they're healed and not outrageously large.
There are plenty of pictures of these activities in BME/HARD if that's what you're looking for.