I have ordered a new pink-coated silver nose ring to replace the current one, because the pink has flaked off and the silver tarnished black beneath it, which looks really bad.
To prevent that happening with the new one, I’m thinking of coating it in cyanoacrylate glue (and letting it dry) before I insert it. Should at least give me longer wear on the pink. Once it’s in, it’ll stay in for the duration.
But, is dried superglue completely inert? No harmful whatever leeching into me?
Currently used extensively for embolization of vessels, and to close wounds. Has to be inert and harmless.
Yep, it can be used in the human body, doctors and dentists sometimes use it in place of sutures, and so forth. It shouldn’t be harmful to you.
However, as someone who has extensive experience in working with superglue, the dried form can be rough. After you coat the nosering you may want to sand it lightly with very fine sandpaper to eliminate any roughness.
And, of course, if irritation occurs discontinue wearing the ring. A quirky reaction is always possible.
One danger is the reaction with cotton. Do not accidentally glue a cotton ball onto your face !
Applying cyanoacrylate to some materials made of cotton or wool (such as cotton swabs, cotton balls, and certain yarns or fabrics) results in a powerful, rapid exothermic reaction. The heat released may cause serious burns,[19] ignite the cotton product, or release irritating white smoke. Material Safety Data Sheets for cyanoacrylate instruct users not to wear cotton or wool clothing, especially cotton gloves, when applying or handling cyanoacrylates.
Some people are allergic to cyanoacrylates - most commonly people who work with it a lot, as they have become sensitised.
I do not know, but am thinking that perhaps occasionally spraying it with hairspray would work. Would give a thin coat of varnish via a product used on the human body for a long time.
Is super glue nice and transparent and shiny?
I would try a layer of clear nail polish, myself.
This seems like a bad idea. Hairspray is not going to provide any real protection, but it will make the thing nice and sticky so that all kinds of crud will probably stick to it.
I don’t really think super glue is a great choice either. If you have to coat it in something, clear nail polish might work, but I don’t know if it will be entirely skin-friendly for long-term placement in a piercing. I’d just buy a high-quality piece of body jewelry (perhaps anodized in pink) from a real body jewelry seller (not a store at the mall that sells cheap junk) and leave it alone, myself.
Cured superglue is essentially acrylic. It is unlikely to be a suitable material for your use - it is quite brittle, and not really all that strong.
As noted above, you are far better off purchasing a proper piece of well made jewellery. Good silver articles are rhodium plated. Rhodium is a platinoid (same chemical family as platinum) and is thus very inert, and has a very similar lustre to silver. It wont tarnish. As to the pink coating, you will get what you pay for. I would imagine that there may be high quality fired enamels that might be used.
One of my model-building buddies found out about this when he spilled most of a tube of superglue into his lap. Not permanently hurt, but apparently it got pretty exciting for a while. 
I don’t feel so bad about Supergluing devil horns onto my head that one Halloween twenty years ago. Girls would pull on them and they wouldn’t come off so they would ask how it was attached? I offered them another appendage to examine and see if they could figure out how it was attached . . . a then did my best devilish laugh. 
Superglue is not all that clear and and coating a material with it will result in a dull, hazy film which I doubt is your objective. This is a bad idea. Get a decent nose ring.
I was using super glue to attach feathers to an arrow, the reaction caused fumes that burned my lungs and took my breath away. Pretty scary and it came on very quickly.
Here are some finishes/glosses that are meant to be painted on to painted items in order to protect the paint and still look nice. I have absolutely no idea how safe they would be in the application you describe. Maybe others could comment, but I’m fairly sure it would work in terms of protection and looks: